diff --git a/lsm/marshal.go b/lsm/marshal.go
new file mode 100644
index 0000000..00034d2
--- /dev/null
+++ b/lsm/marshal.go
@@ -0,0 +1,138 @@
+package lsm
+
+import (
+	"bytes"
+	"encoding"
+	"encoding/binary"
+	"fmt"
+)
+
+type entry struct {
+	key   string
+	value uint64
+}
+
+// MarshalBinary implements encoding.BinaryMarshaler.
+func (e *entry) MarshalBinary() (data []byte, err error) {
+	data = make([]byte, len(e.key), len(e.key)+binary.MaxVarintLen16)
+	copy(data, e.key)
+
+	data = binary.AppendUvarint(data, e.value)
+	reverse(data[len(e.key):])
+	return data, err
+}
+
+// UnmarshalBinary implements encoding.BinaryUnmarshaler.
+func (e *entry) UnmarshalBinary(data []byte) error {
+	// fmt.Println("unmarshal", data, string(data))
+
+	if len(data) < binary.MaxVarintLen16 {
+		return fmt.Errorf("%w: bad data", ErrDecode)
+	}
+	head := make([]byte, binary.MaxVarintLen16)
+	copy(head, data[max(0, len(data)-cap(head)):])
+	reverse(head)
+
+	size := 0
+	e.value, size = binary.Uvarint(head)
+	if size == 0 {
+		return fmt.Errorf("%w: invalid data", ErrDecode)
+	}
+	e.key = string(data[:len(data)-size])
+
+	return nil
+}
+
+var _ encoding.BinaryMarshaler = (*entry)(nil)
+var _ encoding.BinaryUnmarshaler = (*entry)(nil)
+
+type entries []entry
+
+// MarshalBinary implements encoding.BinaryMarshaler.
+func (lis *entries) MarshalBinary() (data []byte, err error) {
+	var buf bytes.Buffer
+
+	for _, e := range *lis {
+		d, err := e.MarshalBinary()
+		if err != nil {
+			return nil, err
+		}
+
+		_, err = buf.Write(d)
+		if err != nil {
+			return nil, err
+		}
+
+		_, err = buf.Write(reverse(binary.AppendUvarint(make([]byte, 0, binary.MaxVarintLen32), uint64(len(d)))))
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return buf.Bytes(), err
+}
+
+// UnmarshalBinary implements encoding.BinaryUnmarshaler.
+func (lis *entries) UnmarshalBinary(data []byte) error {
+	head := make([]byte, binary.MaxVarintLen16)
+	pos := uint64(len(data))
+
+	for pos > 0 {
+		copy(head, data[max(0, pos-uint64(cap(head))):])
+		length, size := binary.Uvarint(reverse(head))
+
+		e := entry{}
+		if err := e.UnmarshalBinary(data[max(0, pos-(length+uint64(size))) : pos-uint64(size)]); err != nil {
+			return err
+		}
+		*lis = append(*lis, e)
+
+		pos -= length + uint64(size)
+	}
+	reverse(*lis)
+	return nil
+}
+
+var _ encoding.BinaryMarshaler = (*entries)(nil)
+var _ encoding.BinaryUnmarshaler = (*entries)(nil)
+
+type segment struct {
+	entries entries
+}
+
+// MarshalBinary implements encoding.BinaryMarshaler.
+func (s *segment) MarshalBinary() (data []byte, err error) {
+	head := header{
+		entries: uint64(len(s.entries)),
+	}
+
+	data, err = s.entries.MarshalBinary()
+	if err != nil {
+		return nil, err
+	}
+
+	head.datalen = uint64(len(data))
+
+	h := hash()
+	h.Write(data)
+	head.sig = h.Sum(nil)
+
+	return head.Append(data), err
+}
+
+// UnmarshalBinary implements encoding.BinaryUnmarshaler.
+func (s *segment) UnmarshalBinary(data []byte) error {
+	head, err := ReadHead(data)
+	if err != nil {
+		return err
+	}
+
+	h := hash()
+	h.Write(data[:head.datalen])
+	if !bytes.Equal(head.sig, h.Sum(nil)) {
+		return fmt.Errorf("%w: invalid checksum", ErrDecode)
+	}
+
+	s.entries = make(entries, 0, head.entries)
+	return s.entries.UnmarshalBinary(data[:head.datalen])
+}
diff --git a/lsm/marshal_test.go b/lsm/marshal_test.go
new file mode 100644
index 0000000..e67f0a0
--- /dev/null
+++ b/lsm/marshal_test.go
@@ -0,0 +1,71 @@
+package lsm
+
+import (
+	"io/fs"
+	"testing"
+
+	"github.com/matryer/is"
+)
+
+func TestEncoding(t *testing.T) {
+	is := is.New(t)
+
+	data := segment{entries: entries{
+		{"key-1", 1},
+		{"key-2", 2},
+		{"key-3", 3},
+		{"longerkey-4", 65535},
+	}}
+
+	b, err := data.MarshalBinary()
+	is.NoErr(err)
+
+	var got segment
+	err = got.UnmarshalBinary(b)
+	is.NoErr(err)
+
+	is.Equal(data, got)
+}
+
+func TestReverse(t *testing.T) {
+	is := is.New(t)
+
+	got := []byte("gnirts a si siht")
+	reverse(got)
+
+	is.Equal(got, []byte("this is a string"))
+
+	got = []byte("!gnirts a si siht")
+	reverse(got)
+
+	is.Equal(got, []byte("this is a string!"))
+}
+
+func TestFile(t *testing.T) {
+	is := is.New(t)
+
+	f := basicFile(t)
+
+	sf, err := ReadFile(f)
+	is.NoErr(err)
+
+	is.Equal(len(sf.segments), 3)
+}
+
+func basicFile(t *testing.T) fs.File {
+	t.Helper()
+
+	data := segment{entries: entries{
+		{"key-1", 1},
+		{"key-2", 2},
+		{"key-3", 3},
+		{"longerkey-4", 65535},
+	}}
+
+	b, err := data.MarshalBinary()
+	if err != nil {
+		t.Error(err)
+	}
+
+	return NewFile(b, b, b)
+}
diff --git a/lsm/sst.go b/lsm/sst.go
new file mode 100644
index 0000000..bc901c7
--- /dev/null
+++ b/lsm/sst.go
@@ -0,0 +1,315 @@
+// SPDX-FileCopyrightText: 2023 Jon Lundy <jon@xuu.cc>
+// SPDX-License-Identifier: BSD-3-Clause
+package lsm
+
+import (
+	"bytes"
+	"encoding"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"hash/fnv"
+	"io"
+	"io/fs"
+	"sort"
+)
+
+var (
+	magic      = reverse(append([]byte("Souris"), '\x01'))
+	hash       = fnv.New32a
+	hashLength = hash().Size()
+	// segmentSize         = 2 ^ 16 // min 2^9 = 512b, max? 2^20 = 1M
+	segmentFooterLength = len(magic) + hashLength + binary.MaxVarintLen32 + binary.MaxVarintLen32
+)
+
+type header struct {
+	sig     []byte
+	entries uint64
+	datalen uint64
+	headlen uint64
+	end     int64
+}
+
+func ReadHead(data []byte) (*header, error) {
+	if len(data) < len(magic)+6 {
+		return nil, fmt.Errorf("%w: invalid size", ErrDecode)
+	}
+
+	if !bytes.Equal(data[len(data)-len(magic):], magic) {
+		return nil, fmt.Errorf("%w: invalid header", ErrDecode)
+	}
+
+	head := make([]byte, 0, segmentFooterLength)
+	head = reverse(append(head, data[max(0, len(data)-cap(head)-1):]...))
+	size, s := binary.Uvarint(head[len(magic)+4:])
+	length, i := binary.Uvarint(head[len(magic)+4+s:])
+
+	return &header{
+		sig:     head[len(magic) : len(magic)+4],
+		entries: size,
+		datalen: length,
+		headlen: uint64(len(magic) + hashLength + s + i),
+		end:     int64(len(data)),
+	}, nil
+}
+func (h *header) Append(data []byte) []byte {
+
+	length := len(data)
+	data = append(data, h.sig...)
+	data = binary.AppendUvarint(data, h.entries)
+	data = binary.AppendUvarint(data, h.datalen)
+	reverse(data[length:])
+
+	return append(data, magic...)
+}
+
+var _ encoding.BinaryMarshaler = (*segment)(nil)
+var _ encoding.BinaryUnmarshaler = (*segment)(nil)
+
+var ErrDecode = errors.New("decode")
+
+func reverse[T any](b []T) []T {
+	l := len(b)
+	for i := 0; i < l/2; i++ {
+		b[i], b[l-i-1] = b[l-i-1], b[i]
+	}
+	return b
+}
+
+// func clone[T ~[]E, E any](e []E) []E {
+// 	return append(e[0:0:0], e...)
+// }
+
+type entryBytes []byte
+
+func (e entryBytes) KeyValue() ([]byte, uint64) {
+	if len(e) < 2 {
+		return nil, 0
+	}
+	head := reverse(append(e[0:0:0], e[max(0, len(e)-binary.MaxVarintLen64):]...))
+	value, i := binary.Uvarint(head)
+	return append(e[0:0:0], e[:len(e)-i]...), value
+}
+func NewKeyValue(key []byte, val uint64) entryBytes {
+	length := len(key)
+	data := append(key[0:0:0], key...)
+	data = binary.AppendUvarint(data, val)
+	reverse(data[length:])
+
+	return data
+}
+
+type listEntries []entryBytes
+
+func (lis *listEntries) WriteTo(wr io.Writer) (int64, error) {
+	if lis == nil {
+		return 0, nil
+	}
+
+	head := header{
+		entries: uint64(len(*lis)),
+	}
+	h := hash()
+
+	wr = io.MultiWriter(wr, h)
+
+	var i int64
+	for _, b := range *lis {
+		j, err := wr.Write(b)
+		i += int64(j)
+		if err != nil {
+			return i, err
+		}
+
+		j, err = wr.Write(reverse(binary.AppendUvarint(make([]byte, 0, binary.MaxVarintLen32), uint64(len(b)))))
+		i += int64(j)
+		if err != nil {
+			return i, err
+		}
+	}
+	head.datalen = uint64(i)
+	head.sig = h.Sum(nil)
+
+	b := head.Append([]byte{})
+	j, err := wr.Write(b)
+	i += int64(j)
+
+	return i, err
+}
+
+var _ sort.Interface = listEntries{}
+
+// Len implements sort.Interface.
+func (lis listEntries) Len() int {
+	return len(lis)
+}
+
+// Less implements sort.Interface.
+func (lis listEntries) Less(i int, j int) bool {
+	iname, _ := lis[i].KeyValue()
+	jname, _ := lis[j].KeyValue()
+
+	return bytes.Compare(iname, jname) < 0
+}
+
+// Swap implements sort.Interface.
+func (lis listEntries) Swap(i int, j int) {
+	lis[i], lis[j] = lis[j], lis[i]
+}
+
+type segmentReader struct {
+	head *header
+	rd   io.ReaderAt
+}
+
+func (s *segmentReader) FirstEntry() (*entryBytes, error) {
+	e, _, err := s.readEntryAt(-1)
+	return e, err
+}
+func (s *segmentReader) Find(needle []byte) (*entryBytes, bool, error) {
+	if s == nil {
+		return nil, false, nil
+	}
+	e, pos, err := s.readEntryAt(-1)
+	if err != nil {
+		return nil, false, err
+	}
+
+	last := e
+	for pos > 0 {
+		key, _ := e.KeyValue()
+		switch bytes.Compare(key, needle) {
+		case 0: // equal
+			return e, true, nil
+		case -1: // key=aaa, needle=bbb
+			last = e
+			e, pos, err = s.readEntryAt(pos)
+			if err != nil {
+				return nil, false, err
+			}
+
+		case 1: // key=ccc, needle=bbb
+			return last, false, nil
+		}
+	}
+	return last, false, nil
+}
+func (s *segmentReader) readEntryAt(pos int64) (*entryBytes, int64, error) {
+	if pos < 0 {
+		pos = s.head.end
+	}
+	head := make([]byte, binary.MaxVarintLen16)
+	s.rd.ReadAt(head, pos-binary.MaxVarintLen16)
+	length, hsize := binary.Uvarint(reverse(head))
+
+	e := make(entryBytes, length)
+	_, err := s.rd.ReadAt(e, pos-int64(length)-int64(hsize))
+
+	return &e, pos - int64(length) - int64(hsize), err
+}
+
+type logFile struct {
+	rd       interface{io.ReaderAt; io.WriterTo}
+	segments []segmentReader
+
+	fs.File
+}
+
+func ReadFile(fd fs.File) (*logFile, error) {
+	l := &logFile{File: fd}
+
+	stat, err := fd.Stat()
+	if err != nil {
+		return nil, err
+	}
+
+	eof := stat.Size()
+	if rd, ok := fd.(interface{io.ReaderAt; io.WriterTo}); ok {
+		l.rd = rd
+
+	} else {
+		rd, err := io.ReadAll(fd)
+		if err != nil {
+			return nil, err
+		}
+		l.rd = bytes.NewReader(rd)
+	}
+
+	for eof > 0 {
+		head := make([]byte, segmentFooterLength)
+		_, err = l.rd.ReadAt(head, eof-int64(segmentFooterLength))
+		if err != nil {
+			return nil, err
+		}
+
+		s := segmentReader{
+			rd: l.rd,
+		}
+		s.head, err = ReadHead(head)
+		s.head.end = eof - int64(s.head.headlen)
+		if err != nil {
+			return nil, err
+		}
+		eof -= int64(s.head.datalen) + int64(s.head.headlen)
+		l.segments = append(l.segments, s)
+	}
+
+	return l, nil
+}
+
+func (l *logFile) Count() int64 {
+	return int64(len(l.segments))
+}
+func (l *logFile) LoadSegment(pos int64) (*segmentBytes, error) {
+	if pos < 0 {
+		pos = int64(len(l.segments) - 1)
+	}
+	if pos > int64(len(l.segments)-1) {
+		return nil, ErrDecode
+	}
+	s := l.segments[pos]
+
+	b := make([]byte, s.head.datalen+s.head.headlen)
+	_, err := l.rd.ReadAt(b, s.head.end-int64(len(b)))
+	if err != nil {
+		return nil, err
+	}
+
+	return &segmentBytes{b, -1}, nil
+}
+func (l *logFile) Find(needle []byte) (*entryBytes, bool, error) {
+	var last segmentReader
+
+	for _, s := range l.segments {
+		e, err := s.FirstEntry()
+		if err != nil {
+			return nil, false, err
+		}
+		k, _ := e.KeyValue()
+		if bytes.Compare(k, needle) > 0 {
+			break
+		}
+		last = s
+	}
+
+	return last.Find(needle)
+}
+func (l *logFile) WriteTo(w io.Writer) (int64, error) {
+	return l.rd.WriteTo(w)
+}
+
+type segmentBytes struct {
+	b   []byte
+	pos int
+}
+
+type dataset struct {
+	rd    io.ReaderAt
+	files []logFile
+
+	fs.FS
+}
+
+func ReadDataset(fd fs.FS) (*dataset, error) {
+	panic("not implemented")
+}
diff --git a/lsm/sst_test.go b/lsm/sst_test.go
new file mode 100644
index 0000000..839a924
--- /dev/null
+++ b/lsm/sst_test.go
@@ -0,0 +1,302 @@
+// SPDX-FileCopyrightText: 2023 Jon Lundy <jon@xuu.cc>
+// SPDX-License-Identifier: BSD-3-Clause
+package lsm
+
+import (
+	"bytes"
+	crand "crypto/rand"
+	"encoding/base64"
+	"io"
+	"io/fs"
+	"math/rand"
+	"os"
+	"sort"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/matryer/is"
+)
+
+func TestLargeFile(t *testing.T) {
+	is := is.New(t)
+
+	segCount := 4098
+
+	f := randFile(t, 2_000_000, segCount)
+
+	sf, err := ReadFile(f)
+	is.NoErr(err)
+
+	is.True(len(sf.segments) <= segCount)
+	var needle []byte
+	for i, s := range sf.segments {
+		e, err := s.FirstEntry()
+		is.NoErr(err)
+		k, v := e.KeyValue()
+		needle = k
+		t.Logf("Segment-%d: %s = %d", i, k, v)
+	}
+	t.Log(f.Stat())
+
+	tt, ok, err := sf.Find(needle)
+	is.NoErr(err)
+	is.True(ok)
+	key, val := tt.KeyValue()
+	t.Log(string(key), val)
+
+	tt, ok, err = sf.Find([]byte("needle"))
+	is.NoErr(err)
+	is.True(!ok)
+	key, val = tt.KeyValue()
+	t.Log(string(key), val)
+
+	tt, ok, err = sf.Find([]byte{'\xff'})
+	is.NoErr(err)
+	is.True(!ok)
+	key, val = tt.KeyValue()
+	t.Log(string(key), val)
+}
+
+func TestLargeFileDisk(t *testing.T) {
+	is := is.New(t)
+
+	segCount := 4098
+
+	t.Log("generate large file")
+	f := randFile(t, 2_000_000, segCount)
+
+	fd, err := os.CreateTemp("", "sst*")
+	is.NoErr(err)
+	defer func() { t.Log("cleanup:", fd.Name()); fd.Close(); os.Remove(fd.Name()) }()
+
+	t.Log("write file:", fd.Name())
+	_, err = io.Copy(fd, f)
+	is.NoErr(err)
+	fd.Seek(0, 0)
+
+	sf, err := ReadFile(fd)
+	is.NoErr(err)
+
+	is.True(len(sf.segments) <= segCount)
+	var needle []byte
+	for i, s := range sf.segments {
+		e, err := s.FirstEntry()
+		is.NoErr(err)
+		k, v := e.KeyValue()
+		needle = k
+		t.Logf("Segment-%d: %s = %d", i, k, v)
+	}
+	t.Log(f.Stat())
+
+	tt, ok, err := sf.Find(needle)
+	is.NoErr(err)
+	is.True(ok)
+	key, val := tt.KeyValue()
+	t.Log(string(key), val)
+
+	tt, ok, err = sf.Find([]byte("needle"))
+	is.NoErr(err)
+	is.True(!ok)
+	key, val = tt.KeyValue()
+	t.Log(string(key), val)
+
+	tt, ok, err = sf.Find([]byte{'\xff'})
+	is.NoErr(err)
+	is.True(!ok)
+	key, val = tt.KeyValue()
+	t.Log(string(key), val)
+}
+
+func BenchmarkLargeFile(b *testing.B) {
+	segCount := 4098 / 4
+	f := randFile(b, 2_000_000, segCount)
+
+	sf, err := ReadFile(f)
+	if err != nil {
+		b.Error(err)
+	}
+	key := make([]byte, 5)
+	keys := make([][]byte, b.N)
+	for i := range keys {
+		_, err = crand.Read(key)
+		if err != nil {
+			b.Error(err)
+		}
+		keys[i] = []byte(base64.RawURLEncoding.EncodeToString(key))
+	}
+	b.Log("ready", b.N)
+	b.ResetTimer()
+	okays := 0
+	each := b.N / 10
+	for n := 0; n < b.N; n++ {
+		if each > 0 && n%each == 0 {
+			b.Log(n)
+		}
+		_, ok, err := sf.Find(keys[n])
+		if err != nil {
+			b.Error(err)
+		}
+		if ok {
+			okays++
+		}
+	}
+	b.Log("okays=", b.N, okays)
+}
+
+func BenchmarkLargeFileB(b *testing.B) {
+	segCount := 4098 / 16
+	f := randFile(b, 2_000_000, segCount)
+
+	sf, err := ReadFile(f)
+	if err != nil {
+		b.Error(err)
+	}
+	key := make([]byte, 5)
+	keys := make([][]byte, b.N)
+	for i := range keys {
+		_, err = crand.Read(key)
+		if err != nil {
+			b.Error(err)
+		}
+		keys[i] = []byte(base64.RawURLEncoding.EncodeToString(key))
+	}
+	b.Log("ready", b.N)
+	b.ResetTimer()
+	okays := 0
+	each := b.N / 10
+	for n := 0; n < b.N; n++ {
+		if each > 0 && n%each == 0 {
+			b.Log(n)
+		}
+		_, ok, err := sf.Find(keys[n])
+		if err != nil {
+			b.Error(err)
+		}
+		if ok {
+			okays++
+		}
+	}
+	b.Log("okays=", b.N, okays)
+}
+
+func randFile(t interface {
+	Helper()
+	Error(...any)
+}, size int, segments int) fs.File {
+	t.Helper()
+
+	lis := make(listEntries, size)
+	for i := range lis {
+		key := make([]byte, 5)
+		_, err := crand.Read(key)
+		if err != nil {
+			t.Error(err)
+		}
+		key = []byte(base64.RawURLEncoding.EncodeToString(key))
+		// key := []byte(fmt.Sprintf("key-%05d", i))
+
+		lis[i] = NewKeyValue(key, rand.Uint64()%16_777_216)
+	}
+
+	sort.Sort(sort.Reverse(&lis))
+	each := size / segments
+	if size%segments != 0 {
+		each++
+	}
+	split := make([]listEntries, segments)
+
+	for i := range split {
+		if (i+1)*each > len(lis) {
+			split[i] = lis[i*each : i*each+len(lis[i*each:])]
+			split = split[:i+1]
+			break
+		}
+		split[i] = lis[i*each : (i+1)*each]
+	}
+
+	var b bytes.Buffer
+	for _, s := range split {
+		s.WriteTo(&b)
+	}
+
+	return NewFile(b.Bytes())
+}
+
+type fakeStat struct {
+	size int64
+}
+
+// IsDir implements fs.FileInfo.
+func (*fakeStat) IsDir() bool {
+	panic("unimplemented")
+}
+
+// ModTime implements fs.FileInfo.
+func (*fakeStat) ModTime() time.Time {
+	panic("unimplemented")
+}
+
+// Mode implements fs.FileInfo.
+func (*fakeStat) Mode() fs.FileMode {
+	panic("unimplemented")
+}
+
+// Name implements fs.FileInfo.
+func (*fakeStat) Name() string {
+	panic("unimplemented")
+}
+
+// Size implements fs.FileInfo.
+func (s *fakeStat) Size() int64 {
+	return s.size
+}
+
+// Sys implements fs.FileInfo.
+func (*fakeStat) Sys() any {
+	panic("unimplemented")
+}
+
+var _ fs.FileInfo = (*fakeStat)(nil)
+
+type rd interface {
+	io.ReaderAt
+	io.Reader
+}
+type fakeFile struct {
+	stat func() fs.FileInfo
+
+	rd
+}
+
+func (fakeFile) Close() error                 { return nil }
+func (f fakeFile) Stat() (fs.FileInfo, error) { return f.stat(), nil }
+
+func NewFile(b ...[]byte) fs.File {
+	in := bytes.Join(b, nil)
+	rd := bytes.NewReader(in)
+	size := int64(len(in))
+	return &fakeFile{stat: func() fs.FileInfo { return &fakeStat{size: size} }, rd: rd}
+}
+func NewFileFromReader(rd *bytes.Reader) fs.File {
+	return &fakeFile{stat: func() fs.FileInfo { return &fakeStat{size: int64(rd.Len())} }, rd: rd}
+}
+
+type fakeFS struct {
+	files map[string]*fakeFile
+	mu    sync.RWMutex
+}
+
+// Open implements fs.FS.
+func (f *fakeFS) Open(name string) (fs.File, error) {
+	f.mu.RLock()
+	defer f.mu.RUnlock()
+
+	if file, ok := f.files[name]; ok {
+		return file, nil
+	}
+
+	return nil, fs.ErrNotExist
+}
+
+var _ fs.FS = (*fakeFS)(nil)