Immutable blocks with mmap reads

Flush sealed head chunks to ULID-named block directories on disk. Each
block contains CRC'd chunk segment files (mmap'd for reads), a binary
index (symbol table, series, postings with TOC), and a meta.json written
last as the immutability gate. WAL is truncated after block fsync,
preserving the crash-safety ording invariant.

internal/block/block_test.go

@@ -0,0 +1,332 @@
+package block
+
+import (
+	"math"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"git.dvdt.dev/david/ingot/internal/chunkenc"
+	"git.dvdt.dev/david/ingot/labels"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type sample struct {
+	t     int64
+	vBits uint64
+}
+
+func s(t int64, v float64) sample { return sample{t, math.Float64bits(v)} }
+
+// makeChunk creates a chunk with the given samples and returns its raw bytes.
+func makeChunk(t *testing.T, samples []sample) []byte {
+	t.Helper()
+	c := chunkenc.NewXORChunk()
+	a, err := c.Appender()
+	require.NoError(t, err)
+	for _, s := range samples {
+		a.Append(s.t, math.Float64frombits(s.vBits))
+	}
+	return append([]byte(nil), c.Bytes()...)
+}
+
+// collectIterator reads all samples from a ChunkIterator.
+func collectIterator(t *testing.T, it chunkenc.ChunkIterator) []sample {
+	t.Helper()
+	var out []sample
+	for it.Next() {
+		ts, v := it.At()
+		out = append(out, sample{ts, math.Float64bits(v)})
+	}
+	require.NoError(t, it.Err())
+	return out
+}
+
+func TestBlockRoundTrip(t *testing.T) {
+	tests := []struct {
+		name   string
+		series []struct {
+			ref     uint64
+			labels  []labels.Label
+			samples []sample
+		}
+	}{
+		{
+			name: "single_series_single_chunk",
+			series: []struct {
+				ref     uint64
+				labels  []labels.Label
+				samples []sample
+			}{
+				{
+					ref:    1,
+					labels: []labels.Label{{Name: "__name__", Value: "temp"}},
+					samples: []sample{
+						s(1000, 71.3), s(1015, 71.4), s(1030, 71.5),
+					},
+				},
+			},
+		},
+		{
+			name: "multiple_series",
+			series: []struct {
+				ref     uint64
+				labels  []labels.Label
+				samples []sample
+			}{
+				{
+					ref:    1,
+					labels: []labels.Label{{Name: "__name__", Value: "temp"}, {Name: "room", Value: "office"}},
+					samples: []sample{
+						s(1000, 71.3), s(1015, 71.4), s(1030, 71.5),
+					},
+				},
+				{
+					ref:    2,
+					labels: []labels.Label{{Name: "__name__", Value: "humidity"}, {Name: "room", Value: "office"}},
+					samples: []sample{
+						s(1000, 55.0), s(1015, 54.5), s(1030, 54.0),
+					},
+				},
+			},
+		},
+		{
+			name: "large_chunk",
+			series: []struct {
+				ref     uint64
+				labels  []labels.Label
+				samples []sample
+			}{
+				{
+					ref:    1,
+					labels: []labels.Label{{Name: "__name__", Value: "counter"}},
+					samples: func() []sample {
+						rnd := rand.New(rand.NewSource(42))
+						out := make([]sample, 120)
+						ts, v := int64(0), 0.0
+						for i := range out {
+							out[i] = s(ts, v)
+							ts += 15000 + int64(rnd.Intn(100)) - 50
+							v += rnd.Float64()
+						}
+						return out
+					}(),
+				},
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			dataDir := t.TempDir()
+
+			// Build SeriesFlush data.
+			var flushData []SeriesFlush
+			for _, s := range tc.series {
+				data := makeChunk(t, s.samples)
+				minT, maxT := s.samples[0].t, s.samples[len(s.samples)-1].t
+				flushData = append(flushData, SeriesFlush{
+					Ref:    s.ref,
+					Labels: s.labels,
+					Chunks: []ChunkData{{MinT: minT, MaxT: maxT, Data: data}},
+				})
+			}
+
+			// Write block.
+			ulid, err := Flush(dataDir, flushData)
+			require.NoError(t, err)
+			require.NotEmpty(t, ulid)
+
+			// Open block for reading.
+			blockDir := filepath.Join(dataDir, ulid)
+			r, err := Open(blockDir)
+			require.NoError(t, err)
+			defer r.Close()
+
+			// Verify meta.
+			assert.Equal(t, ulid, r.Meta.ULID)
+			assert.Equal(t, 1, r.Meta.Version)
+			assert.Equal(t, len(tc.series), r.Meta.Stats.NumSeries)
+			assert.Equal(t, len(tc.series), r.Meta.Stats.NumChunks)
+
+			// Verify each series' data via iteration.
+			for _, s := range tc.series {
+				it, err := r.SeriesChunkIterator(s.ref, math.MinInt64, math.MaxInt64)
+				require.NoError(t, err)
+				got := collectIterator(t, it)
+				require.Equal(t, len(s.samples), len(got), "ref %d sample count", s.ref)
+				for i, want := range s.samples {
+					assert.Equal(t, want.t, got[i].t, "ref %d sample %d t", s.ref, i)
+					assert.Equal(t, want.vBits, got[i].vBits, "ref %d sample %d v", s.ref, i)
+				}
+			}
+
+			// Verify postings.
+			for _, s := range tc.series {
+				for _, l := range s.labels {
+					refs := r.Postings(l.Name, l.Value)
+					assert.Contains(t, refs, s.ref, "postings for %s=%s", l.Name, l.Value)
+				}
+			}
+
+			// Verify labels lookup.
+			for _, s := range tc.series {
+				ls, ok := r.Labels(s.ref)
+				assert.True(t, ok)
+				assert.Equal(t, s.labels, ls)
+			}
+		})
+	}
+}
+
+func TestBlockMultipleChunksPerSeries(t *testing.T) {
+	dataDir := t.TempDir()
+
+	chunk1Samples := []sample{s(1000, 1.0), s(1015, 2.0), s(1030, 3.0)}
+	chunk2Samples := []sample{s(2000, 4.0), s(2015, 5.0), s(2030, 6.0)}
+	allSamples := append(chunk1Samples, chunk2Samples...)
+
+	flushData := []SeriesFlush{
+		{
+			Ref:    1,
+			Labels: []labels.Label{{Name: "__name__", Value: "temp"}},
+			Chunks: []ChunkData{
+				{MinT: 1000, MaxT: 1030, Data: makeChunk(t, chunk1Samples)},
+				{MinT: 2000, MaxT: 2030, Data: makeChunk(t, chunk2Samples)},
+			},
+		},
+	}
+
+	ulid, err := Flush(dataDir, flushData)
+	require.NoError(t, err)
+
+	r, err := Open(filepath.Join(dataDir, ulid))
+	require.NoError(t, err)
+	defer r.Close()
+
+	// Full range.
+	it, err := r.SeriesChunkIterator(1, math.MinInt64, math.MaxInt64)
+	require.NoError(t, err)
+	got := collectIterator(t, it)
+	require.Equal(t, len(allSamples), len(got))
+	for i, want := range allSamples {
+		assert.Equal(t, want.t, got[i].t, "sample %d t", i)
+		assert.Equal(t, want.vBits, got[i].vBits, "sample %d v", i)
+	}
+
+	// Query only second chunk's range.
+	it, err = r.SeriesChunkIterator(1, 2000, 3000)
+	require.NoError(t, err)
+	got = collectIterator(t, it)
+	require.Equal(t, len(chunk2Samples), len(got))
+	for i, want := range chunk2Samples {
+		assert.Equal(t, want.t, got[i].t, "sample %d t", i)
+		assert.Equal(t, want.vBits, got[i].vBits, "sample %d v", i)
+	}
+
+	// Query with no overlap.
+	it, err = r.SeriesChunkIterator(1, 5000, 6000)
+	require.NoError(t, err)
+	got = collectIterator(t, it)
+	assert.Empty(t, got)
+
+	// Unknown ref.
+	it, err = r.SeriesChunkIterator(999, math.MinInt64, math.MaxInt64)
+	require.NoError(t, err)
+	assert.False(t, it.Next())
+}
+
+func TestBlockMetaTimeBounds(t *testing.T) {
+	dataDir := t.TempDir()
+
+	flushData := []SeriesFlush{
+		{
+			Ref:    1,
+			Labels: []labels.Label{{Name: "__name__", Value: "a"}},
+			Chunks: []ChunkData{{MinT: 500, MaxT: 1000, Data: makeChunk(t, []sample{s(500, 1.0), s(1000, 2.0)})}},
+		},
+		{
+			Ref:    2,
+			Labels: []labels.Label{{Name: "__name__", Value: "b"}},
+			Chunks: []ChunkData{{MinT: 200, MaxT: 800, Data: makeChunk(t, []sample{s(200, 3.0), s(800, 4.0)})}},
+		},
+	}
+
+	ulid, err := Flush(dataDir, flushData)
+	require.NoError(t, err)
+
+	r, err := Open(filepath.Join(dataDir, ulid))
+	require.NoError(t, err)
+	defer r.Close()
+
+	assert.Equal(t, int64(200), r.Meta.MinTime)
+	assert.Equal(t, int64(1000), r.Meta.MaxTime)
+}
+
+func TestULIDRoundTrip(t *testing.T) {
+	for i := 0; i < 100; i++ {
+		u := newULID()
+		assert.Equal(t, 26, len(u), "ULID length")
+
+		// Should parse without error.
+		_, err := parseULID(u)
+		assert.NoError(t, err, "parse ULID %q", u)
+	}
+}
+
+func TestCorruptChunkCRC(t *testing.T) {
+	dataDir := t.TempDir()
+
+	samples := []sample{s(1000, 71.3), s(1015, 71.4)}
+	flushData := []SeriesFlush{
+		{
+			Ref:    1,
+			Labels: []labels.Label{{Name: "__name__", Value: "temp"}},
+			Chunks: []ChunkData{{MinT: 1000, MaxT: 1015, Data: makeChunk(t, samples)}},
+		},
+	}
+
+	ulid, err := Flush(dataDir, flushData)
+	require.NoError(t, err)
+
+	// Open block, find the chunk ref, then corrupt the chunk file.
+	blockDir := filepath.Join(dataDir, ulid)
+	r, err := Open(blockDir)
+	require.NoError(t, err)
+
+	series := r.Series()
+	require.Equal(t, 1, len(series))
+	chunkRef := series[0].Chunks[0].Ref
+	r.Close()
+
+	// Corrupt chunk data on disk.
+	chunkSeg := chunkRef.Segment()
+	chunkPath := filepath.Join(blockDir, chunksDirName, segmentName(int(chunkSeg)))
+
+	// Read, corrupt a data byte, write back.
+	chunkFile, err := readFileBytes(chunkPath)
+	require.NoError(t, err)
+	off := int(chunkRef.Offset()) + chunkEntryHeaderLen + 1 // corrupt a data byte
+	if off < len(chunkFile) {
+		chunkFile[off] ^= 0xFF
+	}
+	require.NoError(t, writeFileBytes(chunkPath, chunkFile))
+
+	// Re-open and try to read the corrupt chunk.
+	r, err = Open(blockDir)
+	require.NoError(t, err)
+	defer r.Close()
+
+	_, err = r.ChunkIterator(chunkRef)
+	assert.Equal(t, ErrCorruptChunk, err)
+}
+
+func readFileBytes(path string) ([]byte, error) {
+	return os.ReadFile(path)
+}
+
+func writeFileBytes(path string, data []byte) error {
+	return os.WriteFile(path, data, 0644)
+}

internal/block/chunk_reader.go

@@ -0,0 +1,174 @@
+package block
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"hash/crc32"
+	"os"
+	"path/filepath"
+	"sort"
+	"syscall"
+
+	"git.dvdt.dev/david/ingot/internal/chunkenc"
+	"git.dvdt.dev/david/ingot/internal/index"
+)
+
+var (
+	ErrInvalidChunkMagic   = errors.New("block: invalid chunk file magic")
+	ErrInvalidChunkVersion = errors.New("block: unsupported chunk file version")
+	ErrCorruptChunk        = errors.New("block: corrupt chunk (CRC mismatch)")
+	ErrChunkNotFound       = errors.New("block: chunk ref out of bounds")
+)
+
+// chunkReader reads chunk data from mmap'd segment files.
+type chunkReader struct {
+	segments map[int][]byte // segment index -> mmap'd data
+}
+
+func newChunkReader(blockDir string) (*chunkReader, error) {
+	chunksDir := filepath.Join(blockDir, chunksDirName)
+	entries, err := os.ReadDir(chunksDir)
+	if err != nil {
+		return nil, err
+	}
+
+	cr := &chunkReader{segments: make(map[int][]byte)}
+
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		idx := parseSegmentName(e.Name())
+		if idx < 0 {
+			continue
+		}
+
+		data, err := mmapFile(filepath.Join(chunksDir, e.Name()))
+		if err != nil {
+			cr.close()
+			return nil, fmt.Errorf("block: mmap segment %s: %w", e.Name(), err)
+		}
+
+		// Validate header.
+		if len(data) < chunkHeaderLen {
+			cr.close()
+			return nil, ErrInvalidChunkMagic
+		}
+		magic := binary.BigEndian.Uint32(data[:4])
+		if magic != chunkMagic {
+			cr.close()
+			return nil, ErrInvalidChunkMagic
+		}
+		if data[4] != chunkVersion {
+			cr.close()
+			return nil, ErrInvalidChunkVersion
+		}
+
+		cr.segments[idx] = data
+	}
+
+	return cr, nil
+}
+
+// chunkData reads the raw chunk bytes at the given ref, validates CRC.
+func (cr *chunkReader) chunkData(ref index.ChunkRef) ([]byte, error) {
+	seg := int(ref.Segment())
+	off := int(ref.Offset())
+
+	data, ok := cr.segments[seg]
+	if !ok {
+		return nil, ErrChunkNotFound
+	}
+
+	if off+chunkEntryHeaderLen > len(data) {
+		return nil, ErrChunkNotFound
+	}
+
+	dataLen := int(binary.BigEndian.Uint32(data[off : off+4]))
+	encoding := data[off+4]
+	off += chunkEntryHeaderLen
+
+	end := off + dataLen + chunkEntryCRCLen
+	if end > len(data) {
+		return nil, ErrChunkNotFound
+	}
+
+	chunkBytes := data[off : off+dataLen]
+	off += dataLen
+
+	// Validate CRC.
+	wantCRC := binary.BigEndian.Uint32(data[off : off+4])
+	crc := crc32.New(castagnoliTable)
+	crc.Write([]byte{encoding})
+	crc.Write(chunkBytes)
+	if crc.Sum32() != wantCRC {
+		return nil, ErrCorruptChunk
+	}
+
+	return chunkBytes, nil
+}
+
+// chunkIterator returns a ChunkIterator for the chunk at the given ref.
+func (cr *chunkReader) chunkIterator(ref index.ChunkRef) (chunkenc.ChunkIterator, error) {
+	data, err := cr.chunkData(ref)
+	if err != nil {
+		return nil, err
+	}
+	return chunkenc.XORChunkFromBytes(data).Iterator(), nil
+}
+
+func (cr *chunkReader) close() error {
+	for _, data := range cr.segments {
+		syscall.Munmap(data)
+	}
+	cr.segments = nil
+	return nil
+}
+
+// segmentIndices returns sorted segment indices.
+func (cr *chunkReader) segmentIndices() []int {
+	idxs := make([]int, 0, len(cr.segments))
+	for idx := range cr.segments {
+		idxs = append(idxs, idx)
+	}
+	sort.Ints(idxs)
+	return idxs
+}
+
+func mmapFile(path string) ([]byte, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	info, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	if info.Size() == 0 {
+		return nil, fmt.Errorf("block: empty file %s", path)
+	}
+
+	data, err := syscall.Mmap(int(f.Fd()), 0, int(info.Size()),
+		syscall.PROT_READ, syscall.MAP_PRIVATE)
+	if err != nil {
+		return nil, err
+	}
+	return data, nil
+}
+
+func parseSegmentName(name string) int {
+	if len(name) != 6 {
+		return -1
+	}
+	n := 0
+	for _, c := range name {
+		if c < '0' || c > '9' {
+			return -1
+		}
+		n = n*10 + int(c-'0')
+	}
+	return n
+}

internal/block/chunk_writer.go

@@ -0,0 +1,133 @@
+package block
+
+import (
+	"encoding/binary"
+	"hash/crc32"
+	"os"
+	"path/filepath"
+
+	"git.dvdt.dev/david/ingot/internal/index"
+)
+
+const (
+	chunkMagic          uint32 = 0x0FACE5C4
+	chunkVersion        byte   = 1
+	chunkHeaderLen             = 5 // 4-byte magic + 1-byte version
+	chunkEntryHeaderLen        = 5 // 4-byte length + 1-byte encoding
+	chunkEntryCRCLen           = 4
+	encodingXOR         byte   = 1
+	chunkSegmentMaxSize        = 512 * 1024 * 1024 // 512 MiB
+	chunksDirName              = "chunks"
+)
+
+var castagnoliTable = crc32.MakeTable(crc32.Castagnoli)
+
+// chunkWriter writes chunk data to segmented chunk files inside a block directory.
+type chunkWriter struct {
+	dir        string
+	segmentIdx int
+	segmentOff int
+	f          *os.File
+}
+
+func newChunkWriter(blockDir string) (*chunkWriter, error) {
+	chunksDir := filepath.Join(blockDir, chunksDirName)
+	if err := os.MkdirAll(chunksDir, 0755); err != nil {
+		return nil, err
+	}
+	cw := &chunkWriter{dir: chunksDir, segmentIdx: 1}
+	if err := cw.newSegment(); err != nil {
+		return nil, err
+	}
+	return cw, nil
+}
+
+func (cw *chunkWriter) newSegment() error {
+	if cw.f != nil {
+		if err := cw.f.Sync(); err != nil {
+			return err
+		}
+		if err := cw.f.Close(); err != nil {
+			return err
+		}
+	}
+
+	name := segmentName(cw.segmentIdx)
+	f, err := os.Create(filepath.Join(cw.dir, name))
+	if err != nil {
+		return err
+	}
+	cw.f = f
+	cw.segmentOff = 0
+
+	// Write chunk file header.
+	var hdr [chunkHeaderLen]byte
+	binary.BigEndian.PutUint32(hdr[:4], chunkMagic)
+	hdr[4] = chunkVersion
+	n, err := f.Write(hdr[:])
+	cw.segmentOff += n
+	return err
+}
+
+// writeChunk writes a chunk entry and returns its ChunkRef.
+// Format: dataLen(4) | encoding(1) | data(dataLen) | CRC32C(4)
+func (cw *chunkWriter) writeChunk(data []byte) (index.ChunkRef, error) {
+	entrySize := chunkEntryHeaderLen + len(data) + chunkEntryCRCLen
+
+	// Rotate if this entry would exceed segment max size.
+	if cw.segmentOff+entrySize > chunkSegmentMaxSize {
+		cw.segmentIdx++
+		if err := cw.newSegment(); err != nil {
+			return 0, err
+		}
+	}
+
+	ref := index.NewChunkRef(uint32(cw.segmentIdx), uint32(cw.segmentOff))
+
+	// Header: length + encoding.
+	var hdr [chunkEntryHeaderLen]byte
+	binary.BigEndian.PutUint32(hdr[:4], uint32(len(data)))
+	hdr[4] = encodingXOR
+	if _, err := cw.f.Write(hdr[:]); err != nil {
+		return 0, err
+	}
+
+	// Chunk data.
+	if _, err := cw.f.Write(data); err != nil {
+		return 0, err
+	}
+
+	// CRC over encoding + data.
+	crc := crc32.New(castagnoliTable)
+	crc.Write(hdr[4:5]) // encoding byte
+	crc.Write(data)
+	var crcBuf [4]byte
+	binary.BigEndian.PutUint32(crcBuf[:], crc.Sum32())
+	if _, err := cw.f.Write(crcBuf[:]); err != nil {
+		return 0, err
+	}
+
+	cw.segmentOff += entrySize
+	return ref, nil
+}
+
+func (cw *chunkWriter) close() error {
+	if cw.f == nil {
+		return nil
+	}
+	if err := cw.f.Sync(); err != nil {
+		cw.f.Close()
+		return err
+	}
+	return cw.f.Close()
+}
+
+func segmentName(idx int) string {
+	var buf [6]byte
+	s := idx
+	for i := 5; i >= 0; i-- {
+		buf[i] = '0' + byte(s%10)
+		s /= 10
+	}
+	return string(buf[:])
+}

internal/block/meta.go

@@ -0,0 +1,55 @@
+package block
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+)
+
+// BlockMeta describes a block on disk.
+type BlockMeta struct {
+	ULID       string         `json:"ulid"`
+	MinTime    int64          `json:"minTime"`
+	MaxTime    int64          `json:"maxTime"`
+	Stats      BlockStats     `json:"stats"`
+	Compaction CompactionInfo `json:"compaction"`
+	Version    int            `json:"version"`
+}
+
+// BlockStats holds summary statistics for a block.
+type BlockStats struct {
+	NumSamples int `json:"numSamples"`
+	NumSeries  int `json:"numSeries"`
+	NumChunks  int `json:"numChunks"`
+}
+
+// CompactionInfo records the block's compaction lineage.
+type CompactionInfo struct {
+	Level   int      `json:"level"`
+	Sources []string `json:"sources"`
+}
+
+const metaFilename = "meta.json"
+
+// readMeta reads a block's meta.json from the given block directory.
+func readMeta(dir string) (BlockMeta, error) {
+	data, err := os.ReadFile(filepath.Join(dir, metaFilename))
+	if err != nil {
+		return BlockMeta{}, err
+	}
+	var m BlockMeta
+	if err := json.Unmarshal(data, &m); err != nil {
+		return BlockMeta{}, err
+	}
+	return m, nil
+}
+
+// writeMeta writes a block's meta.json to the given block directory.
+// This is the last file written when creating a block — the immutability gate.
+func writeMeta(dir string, m BlockMeta) error {
+	data, err := json.MarshalIndent(m, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(filepath.Join(dir, metaFilename), data, 0644)
+}

internal/block/reader.go

@@ -0,0 +1,147 @@
+package block
+
+import (
+	"os"
+	"path/filepath"
+
+	"git.dvdt.dev/david/ingot/internal/chunkenc"
+	"git.dvdt.dev/david/ingot/internal/index"
+	"git.dvdt.dev/david/ingot/labels"
+)
+
+// Reader provides read access to an immutable on-disk block.
+type Reader struct {
+	dir    string
+	Meta   BlockMeta
+	idx    *index.Reader
+	chunks *chunkReader
+}
+
+// Open opens a block directory for reading. Chunk files are mmap'd.
+func Open(dir string) (*Reader, error) {
+	meta, err := readMeta(dir)
+	if err != nil {
+		return nil, err
+	}
+
+	// Read the index file into memory.
+	indexData, err := os.ReadFile(filepath.Join(dir, "index"))
+	if err != nil {
+		return nil, err
+	}
+	idx, err := index.NewReader(indexData)
+	if err != nil {
+		return nil, err
+	}
+
+	cr, err := newChunkReader(dir)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Reader{
+		dir:    dir,
+		Meta:   meta,
+		idx:    idx,
+		chunks: cr,
+	}, nil
+}
+
+// Series returns all series entries from the index.
+func (r *Reader) Series() []index.SeriesEntry {
+	return r.idx.Series()
+}
+
+// SeriesByRef looks up a series by ref.
+func (r *Reader) SeriesByRef(ref uint64) (index.SeriesEntry, bool) {
+	return r.idx.SeriesByRef(ref)
+}
+
+// Postings returns sorted series refs matching label name=value.
+func (r *Reader) Postings(name, value string) []uint64 {
+	return r.idx.Postings(name, value)
+}
+
+// ChunkIterator returns an iterator for a chunk at the given ref.
+func (r *Reader) ChunkIterator(ref index.ChunkRef) (chunkenc.ChunkIterator, error) {
+	return r.chunks.chunkIterator(ref)
+}
+
+// SeriesChunkIterator returns an iterator over all chunks for a series in a
+// time range. Chunks are iterated in order.
+func (r *Reader) SeriesChunkIterator(ref uint64, mint, maxt int64) (chunkenc.ChunkIterator, error) {
+	entry, ok := r.idx.SeriesByRef(ref)
+	if !ok {
+		return &emptyIterator{}, nil
+	}
+
+	var iters []chunkenc.ChunkIterator
+	for _, cm := range entry.Chunks {
+		if cm.MaxT < mint || cm.MinT > maxt {
+			continue
+		}
+		it, err := r.chunks.chunkIterator(cm.Ref)
+		if err != nil {
+			return nil, err
+		}
+		iters = append(iters, it)
+	}
+
+	if len(iters) == 0 {
+		return &emptyIterator{}, nil
+	}
+	if len(iters) == 1 {
+		return iters[0], nil
+	}
+	return &multiIterator{iters: iters}, nil
+}
+
+// Labels returns the labels for a series by ref.
+func (r *Reader) Labels(ref uint64) ([]labels.Label, bool) {
+	entry, ok := r.idx.SeriesByRef(ref)
+	if !ok {
+		return nil, false
+	}
+	return entry.Labels, true
+}
+
+// Close releases all resources (munmaps chunk files).
+func (r *Reader) Close() error {
+	return r.chunks.close()
+}
+
+// multiIterator chains multiple ChunkIterators in order.
+type multiIterator struct {
+	iters []chunkenc.ChunkIterator
+	cur   int
+}
+
+func (m *multiIterator) Next() bool {
+	for m.cur < len(m.iters) {
+		if m.iters[m.cur].Next() {
+			return true
+		}
+		if m.iters[m.cur].Err() != nil {
+			return false
+		}
+		m.cur++
+	}
+	return false
+}
+
+func (m *multiIterator) At() (int64, float64) {
+	return m.iters[m.cur].At()
+}
+
+func (m *multiIterator) Err() error {
+	if m.cur < len(m.iters) {
+		return m.iters[m.cur].Err()
+	}
+	return nil
+}
+
+type emptyIterator struct{}
+
+func (e *emptyIterator) Next() bool        { return false }
+func (e *emptyIterator) At() (int64, float64) { return 0, 0 }
+func (e *emptyIterator) Err() error        { return nil }

internal/block/ulid.go

@@ -0,0 +1,119 @@
+package block
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"errors"
+	"strings"
+	"time"
+)
+
+// ULID encoding alphabet (Crockford's Base32).
+const ulidEncoding = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"
+
+// newULID generates a ULID with the current time and random entropy.
+// Zero external dependencies — the encoding is simple enough to inline.
+func newULID() string {
+	var b [16]byte
+
+	// Timestamp: upper 48 bits = milliseconds since epoch.
+	ms := uint64(time.Now().UnixMilli())
+	b[0] = byte(ms >> 40)
+	b[1] = byte(ms >> 32)
+	b[2] = byte(ms >> 24)
+	b[3] = byte(ms >> 16)
+	b[4] = byte(ms >> 8)
+	b[5] = byte(ms)
+
+	// Randomness: lower 80 bits.
+	rand.Read(b[6:])
+
+	return encodeULID(b)
+}
+
+// parseULID decodes a ULID string back to its 16-byte binary form.
+func parseULID(s string) ([16]byte, error) {
+	var b [16]byte
+	if len(s) != 26 {
+		return b, errInvalidULID
+	}
+
+	// Decode 26 base32 chars -> 130 bits -> 16 bytes + 2 spare bits.
+	var val [26]byte
+	for i := 0; i < 26; i++ {
+		idx := strings.IndexByte(ulidEncoding, s[i])
+		if idx < 0 {
+			// Try lowercase.
+			idx = strings.IndexByte(ulidEncoding, s[i]&^0x20)
+		}
+		if idx < 0 {
+			return b, errInvalidULID
+		}
+		val[i] = byte(idx)
+	}
+
+	// Pack 5-bit groups into bytes.
+	b[0] = val[0]<<5 | val[1]
+	b[1] = val[2]<<3 | val[3]>>2
+	b[2] = val[3]<<6 | val[4]<<1 | val[5]>>4
+	b[3] = val[5]<<4 | val[6]>>1
+	b[4] = val[6]<<7 | val[7]<<2 | val[8]>>3
+	b[5] = val[8]<<5 | val[9]
+	b[6] = val[10]<<3 | val[11]>>2
+	b[7] = val[11]<<6 | val[12]<<1 | val[13]>>4
+	b[8] = val[13]<<4 | val[14]>>1
+	b[9] = val[14]<<7 | val[15]<<2 | val[16]>>3
+	b[10] = val[16]<<5 | val[17]
+	b[11] = val[18]<<3 | val[19]>>2
+	b[12] = val[19]<<6 | val[20]<<1 | val[21]>>4
+	b[13] = val[21]<<4 | val[22]>>1
+	b[14] = val[22]<<7 | val[23]<<2 | val[24]>>3
+	b[15] = val[24]<<5 | val[25]
+
+	return b, nil
+}
+
+// ulidTime extracts the millisecond timestamp from a ULID string.
+func ulidTime(s string) (int64, error) {
+	b, err := parseULID(s)
+	if err != nil {
+		return 0, err
+	}
+	// Timestamp is in the upper 6 bytes.
+	ms := int64(binary.BigEndian.Uint64(append([]byte{0, 0}, b[:6]...)))
+	return ms, nil
+}
+
+func encodeULID(b [16]byte) string {
+	// 16 bytes = 128 bits, encoded in 26 base32 chars (130 bits, 2 spare).
+	var dst [26]byte
+	dst[0] = ulidEncoding[(b[0]&0xE0)>>5]
+	dst[1] = ulidEncoding[b[0]&0x1F]
+	dst[2] = ulidEncoding[(b[1]&0xF8)>>3]
+	dst[3] = ulidEncoding[(b[1]&0x07)<<2|(b[2]&0xC0)>>6]
+	dst[4] = ulidEncoding[(b[2]&0x3E)>>1]
+	dst[5] = ulidEncoding[(b[2]&0x01)<<4|(b[3]&0xF0)>>4]
+	dst[6] = ulidEncoding[(b[3]&0x0F)<<1|(b[4]&0x80)>>7]
+	dst[7] = ulidEncoding[(b[4]&0x7C)>>2]
+	dst[8] = ulidEncoding[(b[4]&0x03)<<3|(b[5]&0xE0)>>5]
+	dst[9] = ulidEncoding[b[5]&0x1F]
+	dst[10] = ulidEncoding[(b[6]&0xF8)>>3]
+	dst[11] = ulidEncoding[(b[6]&0x07)<<2|(b[7]&0xC0)>>6]
+	dst[12] = ulidEncoding[(b[7]&0x3E)>>1]
+	dst[13] = ulidEncoding[(b[7]&0x01)<<4|(b[8]&0xF0)>>4]
+	dst[14] = ulidEncoding[(b[8]&0x0F)<<1|(b[9]&0x80)>>7]
+	dst[15] = ulidEncoding[(b[9]&0x7C)>>2]
+	dst[16] = ulidEncoding[(b[9]&0x03)<<3|(b[10]&0xE0)>>5]
+	dst[17] = ulidEncoding[b[10]&0x1F]
+	dst[18] = ulidEncoding[(b[11]&0xF8)>>3]
+	dst[19] = ulidEncoding[(b[11]&0x07)<<2|(b[12]&0xC0)>>6]
+	dst[20] = ulidEncoding[(b[12]&0x3E)>>1]
+	dst[21] = ulidEncoding[(b[12]&0x01)<<4|(b[13]&0xF0)>>4]
+	dst[22] = ulidEncoding[(b[13]&0x0F)<<1|(b[14]&0x80)>>7]
+	dst[23] = ulidEncoding[(b[14]&0x7C)>>2]
+	dst[24] = ulidEncoding[(b[14]&0x03)<<3|(b[15]&0xE0)>>5]
+	dst[25] = ulidEncoding[b[15]&0x1F]
+	return string(dst[:])
+}
+
+var errInvalidULID = errors.New("block: invalid ULID")

internal/block/writer.go

@@ -0,0 +1,141 @@
+package block
+
+import (
+	"os"
+	"path/filepath"
+
+	"git.dvdt.dev/david/ingot/internal/index"
+	"git.dvdt.dev/david/ingot/labels"
+)
+
+// ChunkData describes a single chunk to be flushed to a block.
+type ChunkData struct {
+	MinT int64
+	MaxT int64
+	Data []byte // raw XOR chunk bytes (including 2-byte sample count header)
+}
+
+// SeriesFlush describes a series with its sealed chunks for block writing.
+type SeriesFlush struct {
+	Ref    uint64
+	Labels []labels.Label
+	Chunks []ChunkData
+}
+
+// Flush writes a new immutable block from the given series data.
+// It creates a ULID-named directory under dataDir containing:
+//   - chunks/ with segment files
+//   - index file
+//   - meta.json (written last as the immutability gate)
+//
+// Returns the block ULID and any error.
+func Flush(dataDir string, series []SeriesFlush) (string, error) {
+	ulid := newULID()
+	blockDir := filepath.Join(dataDir, ulid)
+
+	if err := os.MkdirAll(blockDir, 0755); err != nil {
+		return "", err
+	}
+
+	// Write chunk files and collect index entries.
+	cw, err := newChunkWriter(blockDir)
+	if err != nil {
+		return "", err
+	}
+
+	var (
+		indexEntries []index.SeriesEntry
+		meta         BlockMeta
+	)
+	meta.ULID = ulid
+	meta.Version = 1
+	meta.Compaction = CompactionInfo{Level: 1, Sources: []string{ulid}}
+	meta.MinTime = int64(^uint64(0) >> 1) // max int64
+	meta.MaxTime = int64(0)
+
+	for _, sf := range series {
+		var chunks []index.ChunkMeta
+		for _, cd := range sf.Chunks {
+			ref, err := cw.writeChunk(cd.Data)
+			if err != nil {
+				cw.close()
+				return "", err
+			}
+			chunks = append(chunks, index.ChunkMeta{
+				MinT: cd.MinT,
+				MaxT: cd.MaxT,
+				Ref:  ref,
+			})
+			if cd.MinT < meta.MinTime {
+				meta.MinTime = cd.MinT
+			}
+			if cd.MaxT > meta.MaxTime {
+				meta.MaxTime = cd.MaxT
+			}
+			meta.Stats.NumChunks++
+			// Count samples from the chunk's 2-byte header.
+			if len(cd.Data) >= 2 {
+				meta.Stats.NumSamples += int(uint16(cd.Data[0])<<8 | uint16(cd.Data[1]))
+			}
+		}
+		indexEntries = append(indexEntries, index.SeriesEntry{
+			Ref:    sf.Ref,
+			Labels: sf.Labels,
+			Chunks: chunks,
+		})
+		meta.Stats.NumSeries++
+	}
+
+	if err := cw.close(); err != nil {
+		return "", err
+	}
+
+	// Write index file.
+	indexPath := filepath.Join(blockDir, "index")
+	indexFile, err := os.Create(indexPath)
+	if err != nil {
+		return "", err
+	}
+
+	iw := index.NewWriter(indexFile)
+	for _, e := range indexEntries {
+		iw.AddSeries(e)
+	}
+	if _, err := iw.WriteTo(); err != nil {
+		indexFile.Close()
+		return "", err
+	}
+	if err := indexFile.Sync(); err != nil {
+		indexFile.Close()
+		return "", err
+	}
+	if err := indexFile.Close(); err != nil {
+		return "", err
+	}
+
+	// Fsync the block directory to ensure all files are durable.
+	if err := syncDir(blockDir); err != nil {
+		return "", err
+	}
+
+	// Write meta.json last — the immutability gate.
+	if err := writeMeta(blockDir, meta); err != nil {
+		return "", err
+	}
+
+	// Fsync the data directory so the block directory entry is visible.
+	if err := syncDir(dataDir); err != nil {
+		return "", err
+	}
+
+	return ulid, nil
+}
+
+func syncDir(dir string) error {
+	d, err := os.Open(dir)
+	if err != nil {
+		return err
+	}
+	defer d.Close()
+	return d.Sync()
+}

internal/chunkenc/xor.go

@@ -28,6 +28,15 @@ func NewXORChunk() *XORChunk {
 	return &XORChunk{b: bstream{stream: make([]byte, 2), count: 0}}
 }
 
+// XORChunkFromBytes creates a read-only XORChunk from raw bytes.
+// The data must include the 2-byte sample count header (as returned by Bytes).
+// The returned chunk supports Iterator, NumSamples, and Bytes but not Appender.
+func XORChunkFromBytes(data []byte) *XORChunk {
+	cp := make([]byte, len(data))
+	copy(cp, data)
+	return &XORChunk{b: bstream{stream: cp}}
+}
+
 func (c *XORChunk) NumSamples() int {
 	return int(binary.BigEndian.Uint16(c.b.bytes()))
 }

internal/chunkenc/xor_test.go

@@ -298,6 +298,58 @@ func TestXORChunk(t *testing.T) {
 	}
 }
 
+func TestXORChunkFromBytes(t *testing.T) {
+	rnd := rand.New(rand.NewSource(99))
+
+	tests := []struct {
+		name    string
+		samples [][2]float64
+	}{
+		{"single", [][2]float64{{1000, 71.3}}},
+		{"two", [][2]float64{{1000, 71.3}, {1015, 71.4}}},
+		{"full_chunk", func() [][2]float64 {
+			out := make([][2]float64, 120)
+			ts, v := int64(0), 70.0
+			for i := range out {
+				out[i] = [2]float64{float64(ts), v}
+				ts += 15000 + int64(rnd.Intn(100)) - 50
+				v += rnd.Float64() - 0.5
+			}
+			return out
+		}()},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			orig := NewXORChunk()
+			a, _ := orig.Appender()
+			for _, s := range tc.samples {
+				a.Append(int64(s[0]), s[1])
+			}
+
+			reconstituted := XORChunkFromBytes(orig.Bytes())
+
+			assert.Equal(t, orig.NumSamples(), reconstituted.NumSamples())
+			assert.Equal(t, orig.Bytes(), reconstituted.Bytes())
+
+			// Verify iteration produces identical samples.
+			it := reconstituted.Iterator()
+			for i, s := range tc.samples {
+				assert.True(t, it.Next(), "sample %d", i)
+				gotT, gotV := it.At()
+				assert.Equal(t, int64(s[0]), gotT, "sample %d t", i)
+				assert.Equal(t, math.Float64bits(s[1]), math.Float64bits(gotV), "sample %d v", i)
+			}
+			assert.False(t, it.Next())
+			assert.NoError(t, it.Err())
+
+			// Appender on non-empty reconstituted chunk should fail.
+			_, err := reconstituted.Appender()
+			assert.Error(t, err)
+		})
+	}
+}
+
 func FuzzXORIterator(f *testing.F) {
 	c := NewXORChunk()
 	a, _ := c.Appender()

internal/head/head.go

@@ -4,8 +4,10 @@ package head
 
 import (
 	"fmt"
+	"path/filepath"
 	"sync/atomic"
 
+	"git.dvdt.dev/david/ingot/internal/block"
 	"git.dvdt.dev/david/ingot/internal/chunkenc"
 	"git.dvdt.dev/david/ingot/internal/wal"
 	"git.dvdt.dev/david/ingot/labels"
@@ -13,6 +15,7 @@ import (
 
 // Head is the in-memory store for active series and their chunks.
 type Head struct {
+	dataDir string // parent directory containing WAL and block dirs
 	series  *seriesMap
 	wal     *wal.WAL
 	nextRef atomic.Uint64
@@ -23,6 +26,7 @@ type Head struct {
 }
 
 // Open creates or recovers a Head backed by a WAL in walDir.
+// The dataDir (parent of walDir) is used for writing blocks.
 func Open(walDir string, walOpts wal.Options) (*Head, error) {
 	w, err := wal.Open(walDir, walOpts)
 	if err != nil {
@@ -30,8 +34,9 @@ func Open(walDir string, walOpts wal.Options) (*Head, error) {
 	}
 
 	h := &Head{
-		series: newSeriesMap(),
-		wal:    w,
+		dataDir: filepath.Dir(walDir),
+		series:  newSeriesMap(),
+		wal:     w,
 	}
 
 	if err := h.replay(); err != nil {
@@ -145,3 +150,71 @@ func (h *Head) MaxTime() int64 { return h.maxTime.Load() }
 func (h *Head) Close() error {
 	return h.wal.Close()
 }
+
+// FlushOlderThan collects all sealed chunks with maxT <= threshold from all
+// series, writes them to an immutable block, and truncates the WAL.
+//
+// The ordering invariant is enforced: block fsync -> meta.json write -> WAL truncate.
+// Returns the block ULID (empty string if nothing to flush) and any error.
+func (h *Head) FlushOlderThan(maxT int64) (string, error) {
+	var flushData []block.SeriesFlush
+
+	h.series.forEach(func(s *memSeries) {
+		s.mu.Lock()
+		defer s.mu.Unlock()
+
+		var toFlush []chunkMeta
+		var remaining []chunkMeta
+		for _, cm := range s.sealed {
+			if cm.maxT <= maxT {
+				toFlush = append(toFlush, cm)
+			} else {
+				remaining = append(remaining, cm)
+			}
+		}
+		if len(toFlush) == 0 {
+			return
+		}
+
+		sf := block.SeriesFlush{
+			Ref:    s.ref,
+			Labels: s.labels,
+		}
+		for _, cm := range toFlush {
+			sf.Chunks = append(sf.Chunks, block.ChunkData{
+				MinT: cm.minT,
+				MaxT: cm.maxT,
+				Data: append([]byte(nil), cm.chunk.Bytes()...),
+			})
+		}
+		flushData = append(flushData, sf)
+
+		// Clear flushed chunks from the series.
+		s.sealed = remaining
+	})
+
+	if len(flushData) == 0 {
+		return "", nil
+	}
+
+	// Write block. Flush handles: chunk files + index + fsync + meta.json.
+	ulid, err := block.Flush(h.dataDir, flushData)
+	if err != nil {
+		return "", fmt.Errorf("head: flush block: %w", err)
+	}
+
+	// WAL truncation: safe because the block is fully fsynced.
+	// Truncate all segments below the current one — the flushed data is now
+	// in the block and doesn't need WAL replay.
+	lastSeg := h.wal.LastSegment()
+	if err := h.wal.Truncate(lastSeg); err != nil {
+		return ulid, fmt.Errorf("head: truncate WAL: %w", err)
+	}
+
+	return ulid, nil
+}
+
+// DataDir returns the data directory (parent of WAL dir).
+func (h *Head) DataDir() string {
+	return h.dataDir
+}

internal/head/head_test.go

@@ -2,10 +2,12 @@ package head
 
 import (
 	"math"
+	"os"
 	"path/filepath"
 	"sync"
 	"testing"
 
+	"git.dvdt.dev/david/ingot/internal/block"
 	"git.dvdt.dev/david/ingot/internal/wal"
 	"git.dvdt.dev/david/ingot/labels"
 	"github.com/stretchr/testify/assert"
@@ -432,6 +434,181 @@ func TestWALReplay(t *testing.T) {
 	}
 }
 
+func TestFlushOlderThan(t *testing.T) {
+	tests := []struct {
+		name            string
+		numSamples      int // samples per series (each at 15s intervals)
+		flushMaxT       int64
+		wantBlockSeries int // number of series in the block
+		wantBlockExists bool
+	}{
+		{
+			name:            "flush_sealed_chunks",
+			numSamples:      250, // 2 sealed chunks (120 each) + 10 active
+			flushMaxT:       math.MaxInt64,
+			wantBlockSeries: 1,
+			wantBlockExists: true,
+		},
+		{
+			name:            "nothing_to_flush",
+			numSamples:      50, // only active chunk, no sealed
+			flushMaxT:       math.MaxInt64,
+			wantBlockSeries: 0,
+			wantBlockExists: false,
+		},
+		{
+			name:            "partial_flush_by_time",
+			numSamples:      250,
+			flushMaxT:       120 * 15000, // only flush first sealed chunk
+			wantBlockSeries: 1,
+			wantBlockExists: true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			h := openHead(t)
+
+			// Append samples.
+			app := h.Appender()
+			ref, err := app.Append(0, []labels.Label{{Name: "__name__", Value: "temp"}}, 0, 0)
+			require.NoError(t, err)
+			for i := 1; i < tc.numSamples; i++ {
+				_, err = app.Append(ref, nil, int64(i*15000), float64(i))
+				require.NoError(t, err)
+			}
+			require.NoError(t, app.Commit())
+
+			// Flush.
+			ulid, err := h.FlushOlderThan(tc.flushMaxT)
+			require.NoError(t, err)
+
+			if !tc.wantBlockExists {
+				assert.Empty(t, ulid)
+				return
+			}
+
+			assert.NotEmpty(t, ulid)
+
+			// Verify block exists and is readable.
+			blockDir := filepath.Join(h.DataDir(), ulid)
+			br, err := block.Open(blockDir)
+			require.NoError(t, err)
+			defer br.Close()
+
+			assert.Equal(t, tc.wantBlockSeries, br.Meta.Stats.NumSeries)
+
+			// Verify block data is correct by iterating.
+			if tc.wantBlockSeries > 0 {
+				it, err := br.SeriesChunkIterator(ref, math.MinInt64, math.MaxInt64)
+				require.NoError(t, err)
+				count := 0
+				for it.Next() {
+					count++
+				}
+				require.NoError(t, it.Err())
+				assert.Greater(t, count, 0, "block should contain samples")
+			}
+
+			// Head should still have its active chunk data.
+			allSamples := collectSamples(t, h, ref, math.MinInt64, math.MaxInt64)
+			assert.Greater(t, len(allSamples), 0, "head should still have active chunk")
+		})
+	}
+}
+
+func TestFlushThenContinueAppending(t *testing.T) {
+	h := openHead(t)
+
+	// Append enough to seal two chunks (240 samples), plus a few more.
+	app := h.Appender()
+	ref, err := app.Append(0, []labels.Label{{Name: "__name__", Value: "temp"}}, 0, 0)
+	require.NoError(t, err)
+	for i := 1; i < 250; i++ {
+		_, err = app.Append(ref, nil, int64(i*15000), float64(i))
+		require.NoError(t, err)
+	}
+	require.NoError(t, app.Commit())
+
+	// Flush sealed chunks.
+	ulid, err := h.FlushOlderThan(math.MaxInt64)
+	require.NoError(t, err)
+	require.NotEmpty(t, ulid)
+
+	// Continue appending after flush.
+	app = h.Appender()
+	for i := 250; i < 260; i++ {
+		_, err = app.Append(ref, nil, int64(i*15000), float64(i))
+		require.NoError(t, err)
+	}
+	require.NoError(t, app.Commit())
+
+	// Head should have the active chunk data (unflushed).
+	allSamples := collectSamples(t, h, ref, math.MinInt64, math.MaxInt64)
+	assert.Greater(t, len(allSamples), 0)
+
+	// Block should have the flushed data.
+	blockDir := filepath.Join(h.DataDir(), ulid)
+	br, err := block.Open(blockDir)
+	require.NoError(t, err)
+	defer br.Close()
+
+	it, err := br.SeriesChunkIterator(ref, math.MinInt64, math.MaxInt64)
+	require.NoError(t, err)
+	blockCount := 0
+	for it.Next() {
+		blockCount++
+	}
+	require.NoError(t, it.Err())
+	assert.Equal(t, 240, blockCount, "block should contain 2 sealed chunks of 120 samples each")
+}
+
+func TestFlushWALTruncation(t *testing.T) {
+	dir := t.TempDir()
+	walDir := filepath.Join(dir, "wal")
+
+	h, err := Open(walDir, wal.Options{})
+	require.NoError(t, err)
+
+	// Append enough to seal chunks.
+	app := h.Appender()
+	ref, err := app.Append(0, []labels.Label{{Name: "__name__", Value: "temp"}}, 0, 0)
+	require.NoError(t, err)
+	for i := 1; i < 250; i++ {
+		_, err = app.Append(ref, nil, int64(i*15000), float64(i))
+		require.NoError(t, err)
+	}
+	require.NoError(t, app.Commit())
+
+	// Count WAL segments before flush.
+	walEntries, err := os.ReadDir(walDir)
+	require.NoError(t, err)
+	segsBefore := len(walEntries)
+
+	// Flush.
+	_, err = h.FlushOlderThan(math.MaxInt64)
+	require.NoError(t, err)
+
+	// WAL segments should have been truncated (or at least not grown).
+	walEntries, err = os.ReadDir(walDir)
+	require.NoError(t, err)
+	segsAfter := len(walEntries)
+	assert.LessOrEqual(t, segsAfter, segsBefore, "WAL should be truncated after flush")
+
+	require.NoError(t, h.Close())
+
+	// Re-open: head should recover from WAL (only unflushed data).
+	h2, err := Open(walDir, wal.Options{})
+	require.NoError(t, err)
+	defer h2.Close()
+
+	// The re-opened head should be functional.
+	app = h2.Appender()
+	_, err = app.Append(0, []labels.Label{{Name: "__name__", Value: "new_series"}}, 5000000, 42.0)
+	require.NoError(t, err)
+	require.NoError(t, app.Commit())
+}
+
 func TestConcurrentAppend(t *testing.T) {
 	tests := []struct {
 		name                string

internal/head/striped.go

@@ -70,6 +70,18 @@ func (sm *seriesMap) set(hash uint64, s *memSeries) {
 	rs.mu.Unlock()
 }
 
+// forEach calls fn for every series in the map. The series lock is NOT held.
+func (sm *seriesMap) forEach(fn func(s *memSeries)) {
+	for i := range sm.refStripes {
+		rs := &sm.refStripes[i]
+		rs.mu.RLock()
+		for _, s := range rs.m {
+			fn(s)
+		}
+		rs.mu.RUnlock()
+	}
+}
+
 // remove deletes a series from both maps.
 func (sm *seriesMap) remove(hash uint64, s *memSeries) {
 	hs := &sm.hashStripes[hash%numStripes]

internal/index/index.go

@@ -0,0 +1,54 @@
+// Package index implements the binary index file format for ingot blocks.
+//
+// An index file contains four sections:
+//
+//	[Header 5B] [Symbol Table] [Series] [Postings] [TOC 28B]
+//
+// The TOC at the end stores offsets to each section. Readers seek to the
+// end, read the TOC, and use the offsets to locate each section.
+package index
+
+import (
+	"errors"
+
+	"git.dvdt.dev/david/ingot/labels"
+)
+
+const (
+	indexMagic   uint32 = 0x0FACEDB1
+	indexVersion byte   = 1
+	headerLen           = 5  // 4-byte magic + 1-byte version
+	tocLen              = 28 // 3 offsets (24) + CRC (4)
+)
+
+var (
+	ErrInvalidMagic   = errors.New("index: invalid magic number")
+	ErrInvalidVersion = errors.New("index: unsupported version")
+	ErrCorruptTOC     = errors.New("index: corrupt TOC (CRC mismatch)")
+	ErrCorruptIndex   = errors.New("index: corrupt index data")
+	ErrTooShort       = errors.New("index: file too short")
+)
+
+// ChunkRef encodes a chunk's location as (segment << 32 | offset).
+type ChunkRef uint64
+
+func NewChunkRef(segment, offset uint32) ChunkRef {
+	return ChunkRef(uint64(segment)<<32 | uint64(offset))
+}
+
+func (r ChunkRef) Segment() uint32 { return uint32(r >> 32) }
+func (r ChunkRef) Offset() uint32  { return uint32(r) }
+
+// ChunkMeta describes a chunk's time range and location in a chunk file.
+type ChunkMeta struct {
+	MinT int64
+	MaxT int64
+	Ref  ChunkRef
+}
+
+// SeriesEntry is the input for writing and the output for reading a series.
+type SeriesEntry struct {
+	Ref    uint64
+	Labels []labels.Label
+	Chunks []ChunkMeta
+}

internal/index/index_test.go

@@ -0,0 +1,208 @@
+package index
+
+import (
+	"bytes"
+	"testing"
+
+	"git.dvdt.dev/david/ingot/labels"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func writeIndex(t *testing.T, entries []SeriesEntry) []byte {
+	t.Helper()
+	var buf bytes.Buffer
+	w := NewWriter(&buf)
+	for _, e := range entries {
+		w.AddSeries(e)
+	}
+	_, err := w.WriteTo()
+	require.NoError(t, err)
+	return buf.Bytes()
+}
+
+func TestIndexRoundTrip(t *testing.T) {
+	tests := []struct {
+		name    string
+		entries []SeriesEntry
+	}{
+		{
+			name: "single_series_single_chunk",
+			entries: []SeriesEntry{
+				{
+					Ref:    1,
+					Labels: []labels.Label{{Name: "__name__", Value: "temp"}},
+					Chunks: []ChunkMeta{{MinT: 1000, MaxT: 2000, Ref: NewChunkRef(1, 0)}},
+				},
+			},
+		},
+		{
+			name: "single_series_multiple_chunks",
+			entries: []SeriesEntry{
+				{
+					Ref:    1,
+					Labels: []labels.Label{{Name: "__name__", Value: "temp"}, {Name: "room", Value: "office"}},
+					Chunks: []ChunkMeta{
+						{MinT: 1000, MaxT: 2000, Ref: NewChunkRef(1, 0)},
+						{MinT: 2001, MaxT: 3000, Ref: NewChunkRef(1, 500)},
+						{MinT: 3001, MaxT: 4000, Ref: NewChunkRef(1, 1000)},
+					},
+				},
+			},
+		},
+		{
+			name: "multiple_series",
+			entries: []SeriesEntry{
+				{
+					Ref:    1,
+					Labels: []labels.Label{{Name: "__name__", Value: "temp"}, {Name: "room", Value: "office"}},
+					Chunks: []ChunkMeta{{MinT: 1000, MaxT: 2000, Ref: NewChunkRef(1, 0)}},
+				},
+				{
+					Ref:    2,
+					Labels: []labels.Label{{Name: "__name__", Value: "humidity"}, {Name: "room", Value: "office"}},
+					Chunks: []ChunkMeta{{MinT: 1000, MaxT: 2000, Ref: NewChunkRef(1, 200)}},
+				},
+				{
+					Ref:    3,
+					Labels: []labels.Label{{Name: "__name__", Value: "temp"}, {Name: "room", Value: "kitchen"}},
+					Chunks: []ChunkMeta{{MinT: 1000, MaxT: 2000, Ref: NewChunkRef(1, 400)}},
+				},
+			},
+		},
+		{
+			name:    "empty",
+			entries: nil,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			data := writeIndex(t, tc.entries)
+
+			r, err := NewReader(data)
+			require.NoError(t, err)
+
+			// Verify series count and content.
+			gotSeries := r.Series()
+			require.Equal(t, len(tc.entries), len(gotSeries))
+
+			for i, want := range tc.entries {
+				got := gotSeries[i]
+				assert.Equal(t, want.Ref, got.Ref, "series %d ref", i)
+				assert.Equal(t, want.Labels, got.Labels, "series %d labels", i)
+				require.Equal(t, len(want.Chunks), len(got.Chunks), "series %d chunk count", i)
+				for j, wc := range want.Chunks {
+					assert.Equal(t, wc.MinT, got.Chunks[j].MinT, "series %d chunk %d minT", i, j)
+					assert.Equal(t, wc.MaxT, got.Chunks[j].MaxT, "series %d chunk %d maxT", i, j)
+					assert.Equal(t, wc.Ref, got.Chunks[j].Ref, "series %d chunk %d ref", i, j)
+				}
+
+				// SeriesByRef lookup.
+				byRef, ok := r.SeriesByRef(want.Ref)
+				assert.True(t, ok, "series %d lookup by ref", i)
+				assert.Equal(t, want.Ref, byRef.Ref)
+			}
+
+			// Verify postings.
+			for _, e := range tc.entries {
+				for _, l := range e.Labels {
+					refs := r.Postings(l.Name, l.Value)
+					assert.Contains(t, refs, e.Ref, "postings for %s=%s should contain ref %d", l.Name, l.Value, e.Ref)
+				}
+			}
+
+			// Verify missing lookups return empty/false.
+			_, ok := r.SeriesByRef(999999)
+			assert.False(t, ok)
+			assert.Nil(t, r.Postings("nonexistent", "value"))
+		})
+	}
+}
+
+func TestIndexPostingsSorted(t *testing.T) {
+	entries := []SeriesEntry{
+		{Ref: 5, Labels: []labels.Label{{Name: "room", Value: "office"}}, Chunks: []ChunkMeta{{MinT: 0, MaxT: 1, Ref: 0}}},
+		{Ref: 2, Labels: []labels.Label{{Name: "room", Value: "office"}}, Chunks: []ChunkMeta{{MinT: 0, MaxT: 1, Ref: 0}}},
+		{Ref: 8, Labels: []labels.Label{{Name: "room", Value: "office"}}, Chunks: []ChunkMeta{{MinT: 0, MaxT: 1, Ref: 0}}},
+	}
+
+	data := writeIndex(t, entries)
+	r, err := NewReader(data)
+	require.NoError(t, err)
+
+	refs := r.Postings("room", "office")
+	require.Equal(t, 3, len(refs))
+	assert.Equal(t, uint64(2), refs[0])
+	assert.Equal(t, uint64(5), refs[1])
+	assert.Equal(t, uint64(8), refs[2])
+}
+
+func TestIndexChunkRefEncoding(t *testing.T) {
+	tests := []struct {
+		name    string
+		segment uint32
+		offset  uint32
+	}{
+		{"zero", 0, 0},
+		{"first_segment", 1, 0},
+		{"with_offset", 1, 12345},
+		{"large_values", 100, 536870912},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			ref := NewChunkRef(tc.segment, tc.offset)
+			assert.Equal(t, tc.segment, ref.Segment())
+			assert.Equal(t, tc.offset, ref.Offset())
+		})
+	}
+}
+
+func TestIndexCorruptData(t *testing.T) {
+	tests := []struct {
+		name    string
+		data    []byte
+		wantErr error
+	}{
+		{
+			name:    "too_short",
+			data:    []byte{1, 2, 3},
+			wantErr: ErrTooShort,
+		},
+		{
+			name: "bad_magic",
+			data: func() []byte {
+				d := writeIndex(t, nil)
+				d[0] = 0xFF
+				return d
+			}(),
+			wantErr: ErrInvalidMagic,
+		},
+		{
+			name: "bad_version",
+			data: func() []byte {
+				d := writeIndex(t, nil)
+				d[4] = 99
+				return d
+			}(),
+			wantErr: ErrInvalidVersion,
+		},
+		{
+			name: "corrupt_toc_crc",
+			data: func() []byte {
+				d := writeIndex(t, nil)
+				d[len(d)-1] ^= 0xFF // flip CRC bits
+				return d
+			}(),
+			wantErr: ErrCorruptTOC,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := NewReader(tc.data)
+			assert.Equal(t, tc.wantErr, err)
+		})
+	}
+}

internal/index/reader.go

@@ -0,0 +1,215 @@
+package index
+
+import (
+	"encoding/binary"
+	"hash/crc32"
+
+	"git.dvdt.dev/david/ingot/labels"
+)
+
+// Reader reads an index from a byte slice (typically mmap'd).
+type Reader struct {
+	data []byte
+
+	symbols     []string
+	series      []SeriesEntry
+	seriesByRef map[uint64]int            // ref -> index into series
+	postings    map[labelPair][]uint64    // label pair -> sorted refs
+}
+
+// NewReader parses an index from data.
+func NewReader(data []byte) (*Reader, error) {
+	if len(data) < headerLen+tocLen {
+		return nil, ErrTooShort
+	}
+
+	// Validate header.
+	magic := binary.BigEndian.Uint32(data[:4])
+	if magic != indexMagic {
+		return nil, ErrInvalidMagic
+	}
+	if data[4] != indexVersion {
+		return nil, ErrInvalidVersion
+	}
+
+	// Read TOC from the last 28 bytes.
+	tocStart := len(data) - tocLen
+	toc := data[tocStart:]
+
+	// Validate TOC CRC.
+	wantCRC := binary.BigEndian.Uint32(toc[24:28])
+	gotCRC := crc32.Checksum(toc[:24], castagnoliTable)
+	if gotCRC != wantCRC {
+		return nil, ErrCorruptTOC
+	}
+
+	symbolsOff := int(binary.BigEndian.Uint64(toc[0:8]))
+	seriesOff := int(binary.BigEndian.Uint64(toc[8:16]))
+	postingsOff := int(binary.BigEndian.Uint64(toc[16:24]))
+
+	r := &Reader{
+		data:        data,
+		seriesByRef: make(map[uint64]int),
+		postings:    make(map[labelPair][]uint64),
+	}
+
+	if err := r.readSymbols(symbolsOff); err != nil {
+		return nil, err
+	}
+	if err := r.readSeries(seriesOff); err != nil {
+		return nil, err
+	}
+	if err := r.readPostings(postingsOff, tocStart); err != nil {
+		return nil, err
+	}
+
+	return r, nil
+}
+
+func (r *Reader) readSymbols(off int) error {
+	if off+4 > len(r.data) {
+		return ErrCorruptIndex
+	}
+	numSymbols := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+	off += 4
+
+	r.symbols = make([]string, 0, numSymbols)
+	for i := 0; i < numSymbols; i++ {
+		if off+2 > len(r.data) {
+			return ErrCorruptIndex
+		}
+		slen := int(binary.BigEndian.Uint16(r.data[off : off+2]))
+		off += 2
+		if off+slen > len(r.data) {
+			return ErrCorruptIndex
+		}
+		r.symbols = append(r.symbols, string(r.data[off:off+slen]))
+		off += slen
+	}
+	return nil
+}
+
+func (r *Reader) readSeries(off int) error {
+	if off+4 > len(r.data) {
+		return ErrCorruptIndex
+	}
+	numSeries := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+	off += 4
+
+	r.series = make([]SeriesEntry, 0, numSeries)
+	for i := 0; i < numSeries; i++ {
+		if off+8 > len(r.data) {
+			return ErrCorruptIndex
+		}
+		ref := binary.BigEndian.Uint64(r.data[off : off+8])
+		off += 8
+
+		if off+2 > len(r.data) {
+			return ErrCorruptIndex
+		}
+		numLabels := int(binary.BigEndian.Uint16(r.data[off : off+2]))
+		off += 2
+
+		ls := make([]labels.Label, numLabels)
+		for j := 0; j < numLabels; j++ {
+			if off+8 > len(r.data) {
+				return ErrCorruptIndex
+			}
+			nameIdx := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+			off += 4
+			valueIdx := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+			off += 4
+
+			if nameIdx >= len(r.symbols) || valueIdx >= len(r.symbols) {
+				return ErrCorruptIndex
+			}
+			ls[j] = labels.Label{Name: r.symbols[nameIdx], Value: r.symbols[valueIdx]}
+		}
+
+		if off+4 > len(r.data) {
+			return ErrCorruptIndex
+		}
+		numChunks := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+		off += 4
+
+		chunks := make([]ChunkMeta, numChunks)
+		for j := 0; j < numChunks; j++ {
+			if off+24 > len(r.data) {
+				return ErrCorruptIndex
+			}
+			chunks[j].MinT = int64(binary.BigEndian.Uint64(r.data[off : off+8]))
+			off += 8
+			chunks[j].MaxT = int64(binary.BigEndian.Uint64(r.data[off : off+8]))
+			off += 8
+			chunks[j].Ref = ChunkRef(binary.BigEndian.Uint64(r.data[off : off+8]))
+			off += 8
+		}
+
+		entry := SeriesEntry{Ref: ref, Labels: ls, Chunks: chunks}
+		r.seriesByRef[ref] = len(r.series)
+		r.series = append(r.series, entry)
+	}
+	return nil
+}
+
+func (r *Reader) readPostings(off, limit int) error {
+	if off+4 > limit {
+		return ErrCorruptIndex
+	}
+	numEntries := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+	off += 4
+
+	for i := 0; i < numEntries; i++ {
+		if off+12 > limit {
+			return ErrCorruptIndex
+		}
+		nameIdx := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+		off += 4
+		valueIdx := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+		off += 4
+		numRefs := int(binary.BigEndian.Uint32(r.data[off : off+4]))
+		off += 4
+
+		if nameIdx >= len(r.symbols) || valueIdx >= len(r.symbols) {
+			return ErrCorruptIndex
+		}
+
+		if off+numRefs*8 > limit {
+			return ErrCorruptIndex
+		}
+
+		refs := make([]uint64, numRefs)
+		for j := 0; j < numRefs; j++ {
+			refs[j] = binary.BigEndian.Uint64(r.data[off : off+8])
+			off += 8
+		}
+
+		key := labelPair{r.symbols[nameIdx], r.symbols[valueIdx]}
+		r.postings[key] = refs
+	}
+	return nil
+}
+
+// Symbols returns all symbols in the index.
+func (r *Reader) Symbols() []string {
+	return r.symbols
+}
+
+// Series returns all series entries.
+func (r *Reader) Series() []SeriesEntry {
+	return r.series
+}
+
+// SeriesByRef looks up a series by its ref.
+func (r *Reader) SeriesByRef(ref uint64) (SeriesEntry, bool) {
+	idx, ok := r.seriesByRef[ref]
+	if !ok {
+		return SeriesEntry{}, false
+	}
+	return r.series[idx], true
+}
+
+// Postings returns sorted series refs for the given label pair.
+func (r *Reader) Postings(name, value string) []uint64 {
+	return r.postings[labelPair{name, value}]
+}

internal/index/writer.go

@@ -0,0 +1,273 @@
+package index
+
+import (
+	"encoding/binary"
+	"hash/crc32"
+	"io"
+	"sort"
+
+	"git.dvdt.dev/david/ingot/labels"
+)
+
+var castagnoliTable = crc32.MakeTable(crc32.Castagnoli)
+
+// Writer builds an index file from series data.
+type Writer struct {
+	w   io.Writer
+	off int // bytes written so far
+
+	symbols    []string       // ordered symbol list
+	symbolIdx  map[string]int // string -> index in symbols
+	series     []SeriesEntry
+	postings   map[labelPair][]uint64 // label pair -> sorted series refs
+}
+
+type labelPair struct {
+	name, value string
+}
+
+// NewWriter creates a Writer that writes to w.
+func NewWriter(w io.Writer) *Writer {
+	return &Writer{
+		w:         w,
+		symbolIdx: make(map[string]int),
+		postings:  make(map[labelPair][]uint64),
+	}
+}
+
+// AddSeries adds a series to the index. Series must be added before calling
+// WriteTo. Labels must be sorted.
+func (iw *Writer) AddSeries(entry SeriesEntry) {
+	for _, l := range entry.Labels {
+		iw.addSymbol(l.Name)
+		iw.addSymbol(l.Value)
+	}
+	iw.series = append(iw.series, entry)
+
+	for _, l := range entry.Labels {
+		key := labelPair{l.Name, l.Value}
+		iw.postings[key] = append(iw.postings[key], entry.Ref)
+	}
+}
+
+func (iw *Writer) addSymbol(s string) {
+	if _, ok := iw.symbolIdx[s]; ok {
+		return
+	}
+	idx := len(iw.symbols)
+	iw.symbols = append(iw.symbols, s)
+	iw.symbolIdx[s] = idx
+}
+
+// WriteTo writes the complete index file. Returns bytes written and any error.
+func (iw *Writer) WriteTo() (int, error) {
+	// Sort symbols for deterministic output.
+	sort.Strings(iw.symbols)
+	iw.symbolIdx = make(map[string]int, len(iw.symbols))
+	for i, s := range iw.symbols {
+		iw.symbolIdx[s] = i
+	}
+
+	iw.off = 0
+
+	// Header.
+	if err := iw.writeHeader(); err != nil {
+		return iw.off, err
+	}
+
+	// Symbol table.
+	symbolsOff := iw.off
+	if err := iw.writeSymbols(); err != nil {
+		return iw.off, err
+	}
+
+	// Series.
+	seriesOff := iw.off
+	if err := iw.writeSeries(); err != nil {
+		return iw.off, err
+	}
+
+	// Postings.
+	postingsOff := iw.off
+	if err := iw.writePostings(); err != nil {
+		return iw.off, err
+	}
+
+	// TOC.
+	if err := iw.writeTOC(symbolsOff, seriesOff, postingsOff); err != nil {
+		return iw.off, err
+	}
+
+	return iw.off, nil
+}
+
+func (iw *Writer) write(b []byte) error {
+	n, err := iw.w.Write(b)
+	iw.off += n
+	return err
+}
+
+func (iw *Writer) writeHeader() error {
+	var buf [headerLen]byte
+	binary.BigEndian.PutUint32(buf[:4], indexMagic)
+	buf[4] = indexVersion
+	return iw.write(buf[:])
+}
+
+func (iw *Writer) writeSymbols() error {
+	var buf [4]byte
+	binary.BigEndian.PutUint32(buf[:], uint32(len(iw.symbols)))
+	if err := iw.write(buf[:]); err != nil {
+		return err
+	}
+
+	var lenbuf [2]byte
+	for _, s := range iw.symbols {
+		binary.BigEndian.PutUint16(lenbuf[:], uint16(len(s)))
+		if err := iw.write(lenbuf[:]); err != nil {
+			return err
+		}
+		if err := iw.write([]byte(s)); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (iw *Writer) writeSeries() error {
+	var buf [8]byte
+
+	// numSeries
+	binary.BigEndian.PutUint32(buf[:4], uint32(len(iw.series)))
+	if err := iw.write(buf[:4]); err != nil {
+		return err
+	}
+
+	for _, s := range iw.series {
+		// ref
+		binary.BigEndian.PutUint64(buf[:8], s.Ref)
+		if err := iw.write(buf[:8]); err != nil {
+			return err
+		}
+
+		// numLabels
+		binary.BigEndian.PutUint16(buf[:2], uint16(len(s.Labels)))
+		if err := iw.write(buf[:2]); err != nil {
+			return err
+		}
+
+		// labels as symbol refs
+		for _, l := range s.Labels {
+			binary.BigEndian.PutUint32(buf[:4], uint32(iw.symbolIdx[l.Name]))
+			if err := iw.write(buf[:4]); err != nil {
+				return err
+			}
+			binary.BigEndian.PutUint32(buf[:4], uint32(iw.symbolIdx[l.Value]))
+			if err := iw.write(buf[:4]); err != nil {
+				return err
+			}
+		}
+
+		// numChunks
+		binary.BigEndian.PutUint32(buf[:4], uint32(len(s.Chunks)))
+		if err := iw.write(buf[:4]); err != nil {
+			return err
+		}
+
+		// chunks
+		for _, cm := range s.Chunks {
+			binary.BigEndian.PutUint64(buf[:8], uint64(cm.MinT))
+			if err := iw.write(buf[:8]); err != nil {
+				return err
+			}
+			binary.BigEndian.PutUint64(buf[:8], uint64(cm.MaxT))
+			if err := iw.write(buf[:8]); err != nil {
+				return err
+			}
+			binary.BigEndian.PutUint64(buf[:8], uint64(cm.Ref))
+			if err := iw.write(buf[:8]); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (iw *Writer) writePostings() error {
+	// Sort postings keys for deterministic output.
+	keys := make([]labelPair, 0, len(iw.postings))
+	for k := range iw.postings {
+		keys = append(keys, k)
+	}
+	sort.Slice(keys, func(i, j int) bool {
+		if keys[i].name != keys[j].name {
+			return keys[i].name < keys[j].name
+		}
+		return keys[i].value < keys[j].value
+	})
+
+	var buf [8]byte
+
+	// numEntries
+	binary.BigEndian.PutUint32(buf[:4], uint32(len(keys)))
+	if err := iw.write(buf[:4]); err != nil {
+		return err
+	}
+
+	for _, key := range keys {
+		refs := iw.postings[key]
+
+		// nameSymIdx
+		binary.BigEndian.PutUint32(buf[:4], uint32(iw.symbolIdx[key.name]))
+		if err := iw.write(buf[:4]); err != nil {
+			return err
+		}
+		// valueSymIdx
+		binary.BigEndian.PutUint32(buf[:4], uint32(iw.symbolIdx[key.value]))
+		if err := iw.write(buf[:4]); err != nil {
+			return err
+		}
+
+		// Sort refs.
+		sort.Slice(refs, func(i, j int) bool { return refs[i] < refs[j] })
+
+		// numRefs
+		binary.BigEndian.PutUint32(buf[:4], uint32(len(refs)))
+		if err := iw.write(buf[:4]); err != nil {
+			return err
+		}
+
+		for _, ref := range refs {
+			binary.BigEndian.PutUint64(buf[:8], ref)
+			if err := iw.write(buf[:8]); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (iw *Writer) writeTOC(symbolsOff, seriesOff, postingsOff int) error {
+	var buf [tocLen]byte
+	binary.BigEndian.PutUint64(buf[0:8], uint64(symbolsOff))
+	binary.BigEndian.PutUint64(buf[8:16], uint64(seriesOff))
+	binary.BigEndian.PutUint64(buf[16:24], uint64(postingsOff))
+	crc := crc32.Checksum(buf[:24], castagnoliTable)
+	binary.BigEndian.PutUint32(buf[24:28], crc)
+	return iw.write(buf[:])
+}
+
+// SymbolIndex returns the index of a symbol string, for use in lookups.
+func (iw *Writer) SymbolIndex(s string) (int, bool) {
+	idx, ok := iw.symbolIdx[s]
+	return idx, ok
+}
+
+// labelValues extracts the sorted label set for a series, resolving symbol refs.
+func resolveLabelNames(ls []labels.Label) []labelPair {
+	pairs := make([]labelPair, len(ls))
+	for i, l := range ls {
+		pairs[i] = labelPair{l.Name, l.Value}
+	}
+	return pairs
+}