diff --git a/leveldb/db.go b/leveldb/db.go
index b948214..caf9b07 100644
--- a/leveldb/db.go
+++ b/leveldb/db.go
@@ -275,7 +275,7 @@
 		// We will drop corrupted table.
 		strict = o.GetStrict(opt.StrictRecovery)
 
-		rec   = &sessionRecord{numLevel: o.GetNumLevel()}
+		rec   = &sessionRecord{}
 		bpool = util.NewBufferPool(o.GetBlockSize() + 5)
 	)
 	buildTable := func(iter iterator.Iterator) (tmp storage.File, size int64, err error) {
@@ -450,132 +450,132 @@
 }
 
 func (db *DB) recoverJournal() error {
-	// Get all tables and sort it by file number.
-	journalFiles_, err := db.s.getFiles(storage.TypeJournal)
+	// Get all journals and sort it by file number.
+	allJournalFiles, err := db.s.getFiles(storage.TypeJournal)
 	if err != nil {
 		return err
 	}
-	journalFiles := files(journalFiles_)
-	journalFiles.sort()
+	files(allJournalFiles).sort()
 
-	// Discard older journal.
-	prev := -1
-	for i, file := range journalFiles {
-		if file.Num() >= db.s.stJournalNum {
-			if prev >= 0 {
-				i--
-				journalFiles[i] = journalFiles[prev]
-			}
-			journalFiles = journalFiles[i:]
-			break
-		} else if file.Num() == db.s.stPrevJournalNum {
-			prev = i
+	// Journals that will be recovered.
+	var recJournalFiles []storage.File
+	for _, jf := range allJournalFiles {
+		if jf.Num() >= db.s.stJournalNum || jf.Num() == db.s.stPrevJournalNum {
+			recJournalFiles = append(recJournalFiles, jf)
 		}
 	}
 
-	var jr *journal.Reader
-	var of storage.File
-	var mem *memdb.DB
-	batch := new(Batch)
-	cm := newCMem(db.s)
-	buf := new(util.Buffer)
-	// Options.
-	strict := db.s.o.GetStrict(opt.StrictJournal)
-	checksum := db.s.o.GetStrict(opt.StrictJournalChecksum)
-	writeBuffer := db.s.o.GetWriteBuffer()
-	recoverJournal := func(file storage.File) error {
-		db.logf("journal@recovery recovering @%d", file.Num())
-		reader, err := file.Open()
-		if err != nil {
-			return err
-		}
-		defer reader.Close()
+	var (
+		of  storage.File // Obsolete file.
+		rec = &sessionRecord{}
+	)
 
-		// Create/reset journal reader instance.
-		if jr == nil {
-			jr = journal.NewReader(reader, dropper{db.s, file}, strict, checksum)
-		} else {
-			jr.Reset(reader, dropper{db.s, file}, strict, checksum)
-		}
-
-		// Flush memdb and remove obsolete journal file.
-		if of != nil {
-			if mem.Len() > 0 {
-				if err := cm.flush(mem, 0); err != nil {
-					return err
-				}
-			}
-			if err := cm.commit(file.Num(), db.seq); err != nil {
-				return err
-			}
-			cm.reset()
-			of.Remove()
-			of = nil
-		}
-
-		// Replay journal to memdb.
-		mem.Reset()
-		for {
-			r, err := jr.Next()
-			if err != nil {
-				if err == io.EOF {
-					break
-				}
-				return errors.SetFile(err, file)
-			}
-
-			buf.Reset()
-			if _, err := buf.ReadFrom(r); err != nil {
-				if err == io.ErrUnexpectedEOF {
-					// This is error returned due to corruption, with strict == false.
-					continue
-				} else {
-					return errors.SetFile(err, file)
-				}
-			}
-			if err := batch.memDecodeAndReplay(db.seq, buf.Bytes(), mem); err != nil {
-				if strict || !errors.IsCorrupted(err) {
-					return errors.SetFile(err, file)
-				} else {
-					db.s.logf("journal error: %v (skipped)", err)
-					// We won't apply sequence number as it might be corrupted.
-					continue
-				}
-			}
-
-			// Save sequence number.
-			db.seq = batch.seq + uint64(batch.Len())
-
-			// Flush it if large enough.
-			if mem.Size() >= writeBuffer {
-				if err := cm.flush(mem, 0); err != nil {
-					return err
-				}
-				mem.Reset()
-			}
-		}
-
-		of = file
-		return nil
-	}
-
-	// Recover all journals.
-	if len(journalFiles) > 0 {
-		db.logf("journal@recovery F·%d", len(journalFiles))
+	// Recover journals.
+	if len(recJournalFiles) > 0 {
+		db.logf("journal@recovery F·%d", len(recJournalFiles))
 
 		// Mark file number as used.
-		db.s.markFileNum(journalFiles[len(journalFiles)-1].Num())
+		db.s.markFileNum(recJournalFiles[len(recJournalFiles)-1].Num())
 
-		mem = memdb.New(db.s.icmp, writeBuffer)
-		for _, file := range journalFiles {
-			if err := recoverJournal(file); err != nil {
+		var (
+			// Options.
+			strict      = db.s.o.GetStrict(opt.StrictJournal)
+			checksum    = db.s.o.GetStrict(opt.StrictJournalChecksum)
+			writeBuffer = db.s.o.GetWriteBuffer()
+
+			jr    *journal.Reader
+			mdb   = memdb.New(db.s.icmp, writeBuffer)
+			buf   = &util.Buffer{}
+			batch = &Batch{}
+		)
+
+		for _, jf := range recJournalFiles {
+			db.logf("journal@recovery recovering @%d", jf.Num())
+
+			err := func() error {
+				fr, err := jf.Open()
+				if err != nil {
+					return err
+				}
+				defer fr.Close()
+
+				// Create or reset journal reader instance.
+				if jr == nil {
+					jr = journal.NewReader(fr, dropper{db.s, jf}, strict, checksum)
+				} else {
+					jr.Reset(fr, dropper{db.s, jf}, strict, checksum)
+				}
+
+				// Flush memdb and remove obsolete journal file.
+				if of != nil {
+					if mdb.Len() > 0 {
+						if _, err := db.s.flushMemdb(rec, mdb, -1); err != nil {
+							return err
+						}
+					}
+					rec.setJournalNum(jf.Num())
+					rec.setSeqNum(db.seq)
+					if err := db.s.commit(rec); err != nil {
+						return err
+					}
+					rec.resetAddedTables()
+					of.Remove()
+					of = nil
+				}
+
+				// Replay journal to memdb.
+				mdb.Reset()
+				for {
+					r, err := jr.Next()
+					if err != nil {
+						if err == io.EOF {
+							break
+						}
+						return errors.SetFile(err, jf)
+					}
+
+					buf.Reset()
+					if _, err := buf.ReadFrom(r); err != nil {
+						if err == io.ErrUnexpectedEOF {
+							// This is error returned due to corruption, with strict == false.
+							continue
+						} else {
+							return errors.SetFile(err, jf)
+						}
+					}
+					if err := batch.memDecodeAndReplay(db.seq, buf.Bytes(), mdb); err != nil {
+						if strict || !errors.IsCorrupted(err) {
+							return errors.SetFile(err, jf)
+						} else {
+							db.s.logf("journal error: %v (skipped)", err)
+							// We won't apply sequence number as it might be corrupted.
+							continue
+						}
+					}
+
+					// Save sequence number.
+					db.seq = batch.seq + uint64(batch.Len())
+
+					// Flush it if large enough.
+					if mdb.Size() >= writeBuffer {
+						if _, err := db.s.flushMemdb(rec, mdb, 0); err != nil {
+							return err
+						}
+						mdb.Reset()
+					}
+				}
+
+				of = jf
+				return nil
+			}()
+			if err != nil {
 				return err
 			}
 		}
 
-		// Flush the last journal.
-		if mem.Len() > 0 {
-			if err := cm.flush(mem, 0); err != nil {
+		// Flush the last memdb.
+		if mdb.Len() > 0 {
+			if _, err := db.s.flushMemdb(rec, mdb, 0); err != nil {
 				return err
 			}
 		}
@@ -587,8 +587,10 @@
 	}
 
 	// Commit.
-	if err := cm.commit(db.journalFile.Num(), db.seq); err != nil {
-		// Close journal.
+	rec.setJournalNum(db.journalFile.Num())
+	rec.setSeqNum(db.seq)
+	if err := db.s.commit(rec); err != nil {
+		// Close journal on error.
 		if db.journal != nil {
 			db.journal.Close()
 			db.journalWriter.Close()
@@ -614,7 +616,7 @@
 		}
 		defer m.decref()
 
-		mk, mv, me := m.mdb.Find(ikey)
+		mk, mv, me := m.Find(ikey)
 		if me == nil {
 			ukey, _, kt, kerr := parseIkey(mk)
 			if kerr != nil {
@@ -652,7 +654,7 @@
 		}
 		defer m.decref()
 
-		mk, _, me := m.mdb.Find(ikey)
+		mk, _, me := m.Find(ikey)
 		if me == nil {
 			ukey, _, kt, kerr := parseIkey(mk)
 			if kerr != nil {
diff --git a/leveldb/db_compaction.go b/leveldb/db_compaction.go
index 447407a..3b4c469 100644
--- a/leveldb/db_compaction.go
+++ b/leveldb/db_compaction.go
@@ -11,7 +11,6 @@
 	"time"
 
 	"github.com/syndtr/goleveldb/leveldb/errors"
-	"github.com/syndtr/goleveldb/leveldb/memdb"
 	"github.com/syndtr/goleveldb/leveldb/opt"
 )
 
@@ -62,53 +61,6 @@
 	}
 }
 
-type cMem struct {
-	s     *session
-	level int
-	rec   *sessionRecord
-}
-
-func newCMem(s *session) *cMem {
-	return &cMem{s: s, rec: &sessionRecord{numLevel: s.o.GetNumLevel()}}
-}
-
-func (c *cMem) flush(mem *memdb.DB, level int) error {
-	s := c.s
-
-	// Write memdb to table.
-	iter := mem.NewIterator(nil)
-	defer iter.Release()
-	t, n, err := s.tops.createFrom(iter)
-	if err != nil {
-		return err
-	}
-
-	// Pick level.
-	if level < 0 {
-		v := s.version()
-		level = v.pickLevel(t.imin.ukey(), t.imax.ukey())
-		v.release()
-	}
-	c.rec.addTableFile(level, t)
-
-	s.logf("mem@flush created L%d@%d N·%d S·%s %q:%q", level, t.file.Num(), n, shortenb(int(t.size)), t.imin, t.imax)
-
-	c.level = level
-	return nil
-}
-
-func (c *cMem) reset() {
-	c.rec = &sessionRecord{numLevel: c.s.o.GetNumLevel()}
-}
-
-func (c *cMem) commit(journal, seq uint64) error {
-	c.rec.setJournalNum(journal)
-	c.rec.setSeqNum(seq)
-
-	// Commit changes.
-	return c.s.commit(c.rec)
-}
-
 func (db *DB) compactionError() {
 	var (
 		err     error
@@ -287,21 +239,18 @@
 }
 
 func (db *DB) memCompaction() {
-	mem := db.getFrozenMem()
-	if mem == nil {
+	mdb := db.getFrozenMem()
+	if mdb == nil {
 		return
 	}
-	defer mem.decref()
+	defer mdb.decref()
 
-	c := newCMem(db.s)
-	stats := new(cStatsStaging)
-
-	db.logf("mem@flush N·%d S·%s", mem.mdb.Len(), shortenb(mem.mdb.Size()))
+	db.logf("memdb@flush N·%d S·%s", mdb.Len(), shortenb(mdb.Size()))
 
 	// Don't compact empty memdb.
-	if mem.mdb.Len() == 0 {
-		db.logf("mem@flush skipping")
-		// drop frozen mem
+	if mdb.Len() == 0 {
+		db.logf("memdb@flush skipping")
+		// drop frozen memdb
 		db.dropFrozenMem()
 		return
 	}
@@ -317,13 +266,20 @@
 		return
 	}
 
-	db.compactionTransactFunc("mem@flush", func(cnt *compactionTransactCounter) (err error) {
+	var (
+		rec        = &sessionRecord{}
+		stats      = &cStatsStaging{}
+		flushLevel int
+	)
+
+	db.compactionTransactFunc("memdb@flush", func(cnt *compactionTransactCounter) (err error) {
 		stats.startTimer()
-		defer stats.stopTimer()
-		return c.flush(mem.mdb, -1)
+		flushLevel, err = db.s.flushMemdb(rec, mdb.DB, -1)
+		stats.stopTimer()
+		return
 	}, func() error {
-		for _, r := range c.rec.addedTables {
-			db.logf("mem@flush revert @%d", r.num)
+		for _, r := range rec.addedTables {
+			db.logf("memdb@flush revert @%d", r.num)
 			f := db.s.getTableFile(r.num)
 			if err := f.Remove(); err != nil {
 				return err
@@ -332,20 +288,23 @@
 		return nil
 	})
 
-	db.compactionTransactFunc("mem@commit", func(cnt *compactionTransactCounter) (err error) {
+	db.compactionTransactFunc("memdb@commit", func(cnt *compactionTransactCounter) (err error) {
 		stats.startTimer()
-		defer stats.stopTimer()
-		return c.commit(db.journalFile.Num(), db.frozenSeq)
+		rec.setJournalNum(db.journalFile.Num())
+		rec.setSeqNum(db.frozenSeq)
+		err = db.s.commit(rec)
+		stats.stopTimer()
+		return
 	}, nil)
 
-	db.logf("mem@flush committed F·%d T·%v", len(c.rec.addedTables), stats.duration)
+	db.logf("memdb@flush committed F·%d T·%v", len(rec.addedTables), stats.duration)
 
-	for _, r := range c.rec.addedTables {
+	for _, r := range rec.addedTables {
 		stats.write += r.size
 	}
-	db.compStats[c.level].add(stats)
+	db.compStats[flushLevel].add(stats)
 
-	// Drop frozen mem.
+	// Drop frozen memdb.
 	db.dropFrozenMem()
 
 	// Resume table compaction.
@@ -557,7 +516,7 @@
 func (db *DB) tableCompaction(c *compaction, noTrivial bool) {
 	defer c.release()
 
-	rec := &sessionRecord{numLevel: db.s.o.GetNumLevel()}
+	rec := &sessionRecord{}
 	rec.addCompPtr(c.level, c.imax)
 
 	if !noTrivial && c.trivial() {
diff --git a/leveldb/db_iter.go b/leveldb/db_iter.go
index 011a94a..656ae98 100644
--- a/leveldb/db_iter.go
+++ b/leveldb/db_iter.go
@@ -40,11 +40,11 @@
 	ti := v.getIterators(slice, ro)
 	n := len(ti) + 2
 	i := make([]iterator.Iterator, 0, n)
-	emi := em.mdb.NewIterator(slice)
+	emi := em.NewIterator(slice)
 	emi.SetReleaser(&memdbReleaser{m: em})
 	i = append(i, emi)
 	if fm != nil {
-		fmi := fm.mdb.NewIterator(slice)
+		fmi := fm.NewIterator(slice)
 		fmi.SetReleaser(&memdbReleaser{m: fm})
 		i = append(i, fmi)
 	}
diff --git a/leveldb/db_state.go b/leveldb/db_state.go
index d4db9d6..24671dd 100644
--- a/leveldb/db_state.go
+++ b/leveldb/db_state.go
@@ -15,8 +15,8 @@
 )
 
 type memDB struct {
-	db  *DB
-	mdb *memdb.DB
+	db *DB
+	*memdb.DB
 	ref int32
 }
 
@@ -27,12 +27,12 @@
 func (m *memDB) decref() {
 	if ref := atomic.AddInt32(&m.ref, -1); ref == 0 {
 		// Only put back memdb with std capacity.
-		if m.mdb.Capacity() == m.db.s.o.GetWriteBuffer() {
-			m.mdb.Reset()
-			m.db.mpoolPut(m.mdb)
+		if m.Capacity() == m.db.s.o.GetWriteBuffer() {
+			m.Reset()
+			m.db.mpoolPut(m.DB)
 		}
 		m.db = nil
-		m.mdb = nil
+		m.DB = nil
 	} else if ref < 0 {
 		panic("negative memdb ref")
 	}
@@ -126,7 +126,7 @@
 	}
 	mem = &memDB{
 		db:  db,
-		mdb: mdb,
+		DB:  mdb,
 		ref: 2,
 	}
 	db.mem = mem
diff --git a/leveldb/db_test.go b/leveldb/db_test.go
index 38bfbf1..2f60d17 100644
--- a/leveldb/db_test.go
+++ b/leveldb/db_test.go
@@ -2445,7 +2445,7 @@
 		if err != nil {
 			t.Fatal(err)
 		}
-		rec := &sessionRecord{numLevel: s.o.GetNumLevel()}
+		rec := &sessionRecord{}
 		rec.addTableFile(i, tf)
 		if err := s.commit(rec); err != nil {
 			t.Fatal(err)
@@ -2455,7 +2455,7 @@
 	// Build grandparent.
 	v := s.version()
 	c := newCompaction(s, v, 1, append(tFiles{}, v.tables[1]...))
-	rec := &sessionRecord{numLevel: s.o.GetNumLevel()}
+	rec := &sessionRecord{}
 	b := &tableCompactionBuilder{
 		s:         s,
 		c:         c,
@@ -2479,7 +2479,7 @@
 	// Build level-1.
 	v = s.version()
 	c = newCompaction(s, v, 0, append(tFiles{}, v.tables[0]...))
-	rec = &sessionRecord{numLevel: s.o.GetNumLevel()}
+	rec = &sessionRecord{}
 	b = &tableCompactionBuilder{
 		s:         s,
 		c:         c,
@@ -2523,7 +2523,7 @@
 	// Compaction with transient error.
 	v = s.version()
 	c = newCompaction(s, v, 1, append(tFiles{}, v.tables[1]...))
-	rec = &sessionRecord{numLevel: s.o.GetNumLevel()}
+	rec = &sessionRecord{}
 	b = &tableCompactionBuilder{
 		s:         s,
 		c:         c,
diff --git a/leveldb/db_write.go b/leveldb/db_write.go
index e1cf30c..1e412b3 100644
--- a/leveldb/db_write.go
+++ b/leveldb/db_write.go
@@ -63,24 +63,24 @@
 	return
 }
 
-func (db *DB) flush(n int) (mem *memDB, nn int, err error) {
+func (db *DB) flush(n int) (mdb *memDB, mdbFree int, err error) {
 	delayed := false
 	flush := func() (retry bool) {
 		v := db.s.version()
 		defer v.release()
-		mem = db.getEffectiveMem()
+		mdb = db.getEffectiveMem()
 		defer func() {
 			if retry {
-				mem.decref()
-				mem = nil
+				mdb.decref()
+				mdb = nil
 			}
 		}()
-		nn = mem.mdb.Free()
+		mdbFree = mdb.Free()
 		switch {
 		case v.tLen(0) >= db.s.o.GetWriteL0SlowdownTrigger() && !delayed:
 			delayed = true
 			time.Sleep(time.Millisecond)
-		case nn >= n:
+		case mdbFree >= n:
 			return false
 		case v.tLen(0) >= db.s.o.GetWriteL0PauseTrigger():
 			delayed = true
@@ -90,15 +90,15 @@
 			}
 		default:
 			// Allow memdb to grow if it has no entry.
-			if mem.mdb.Len() == 0 {
-				nn = n
+			if mdb.Len() == 0 {
+				mdbFree = n
 			} else {
-				mem.decref()
-				mem, err = db.rotateMem(n)
+				mdb.decref()
+				mdb, err = db.rotateMem(n)
 				if err == nil {
-					nn = mem.mdb.Free()
+					mdbFree = mdb.Free()
 				} else {
-					nn = 0
+					mdbFree = 0
 				}
 			}
 			return false
@@ -157,18 +157,18 @@
 		}
 	}()
 
-	mem, memFree, err := db.flush(b.size())
+	mdb, mdbFree, err := db.flush(b.size())
 	if err != nil {
 		return
 	}
-	defer mem.decref()
+	defer mdb.decref()
 
 	// Calculate maximum size of the batch.
 	m := 1 << 20
 	if x := b.size(); x <= 128<<10 {
 		m = x + (128 << 10)
 	}
-	m = minInt(m, memFree)
+	m = minInt(m, mdbFree)
 
 	// Merge with other batch.
 drain:
@@ -197,7 +197,7 @@
 		select {
 		case db.journalC <- b:
 			// Write into memdb
-			if berr := b.memReplay(mem.mdb); berr != nil {
+			if berr := b.memReplay(mdb.DB); berr != nil {
 				panic(berr)
 			}
 		case err = <-db.compPerErrC:
@@ -211,7 +211,7 @@
 		case err = <-db.journalAckC:
 			if err != nil {
 				// Revert memdb if error detected
-				if berr := b.revertMemReplay(mem.mdb); berr != nil {
+				if berr := b.revertMemReplay(mdb.DB); berr != nil {
 					panic(berr)
 				}
 				return
@@ -225,7 +225,7 @@
 		if err != nil {
 			return
 		}
-		if berr := b.memReplay(mem.mdb); berr != nil {
+		if berr := b.memReplay(mdb.DB); berr != nil {
 			panic(berr)
 		}
 	}
@@ -233,7 +233,7 @@
 	// Set last seq number.
 	db.addSeq(uint64(b.Len()))
 
-	if b.size() >= memFree {
+	if b.size() >= mdbFree {
 		db.rotateMem(0)
 	}
 	return
@@ -290,9 +290,9 @@
 	}
 
 	// Check for overlaps in memdb.
-	mem := db.getEffectiveMem()
-	defer mem.decref()
-	if isMemOverlaps(db.s.icmp, mem.mdb, r.Start, r.Limit) {
+	mdb := db.getEffectiveMem()
+	defer mdb.decref()
+	if isMemOverlaps(db.s.icmp, mdb.DB, r.Start, r.Limit) {
 		// Memdb compaction.
 		if _, err := db.rotateMem(0); err != nil {
 			<-db.writeLockC
diff --git a/leveldb/session.go b/leveldb/session.go
index b3906f7..f0bba46 100644
--- a/leveldb/session.go
+++ b/leveldb/session.go
@@ -11,10 +11,8 @@
 	"io"
 	"os"
 	"sync"
-	"sync/atomic"
 
 	"github.com/syndtr/goleveldb/leveldb/errors"
-	"github.com/syndtr/goleveldb/leveldb/iterator"
 	"github.com/syndtr/goleveldb/leveldb/journal"
 	"github.com/syndtr/goleveldb/leveldb/opt"
 	"github.com/syndtr/goleveldb/leveldb/storage"
@@ -127,11 +125,16 @@
 		return
 	}
 	defer reader.Close()
-	strict := s.o.GetStrict(opt.StrictManifest)
-	jr := journal.NewReader(reader, dropper{s, m}, strict, true)
 
-	staging := s.stVersion.newStaging()
-	rec := &sessionRecord{numLevel: s.o.GetNumLevel()}
+	var (
+		// Options.
+		numLevel = s.o.GetNumLevel()
+		strict   = s.o.GetStrict(opt.StrictManifest)
+
+		jr      = journal.NewReader(reader, dropper{s, m}, strict, true)
+		rec     = &sessionRecord{}
+		staging = s.stVersion.newStaging()
+	)
 	for {
 		var r io.Reader
 		r, err = jr.Next()
@@ -143,7 +146,7 @@
 			return errors.SetFile(err, m)
 		}
 
-		err = rec.decode(r)
+		err = rec.decode(r, numLevel)
 		if err == nil {
 			// save compact pointers
 			for _, r := range rec.compPtrs {
@@ -206,250 +209,3 @@
 
 	return
 }
-
-// Pick a compaction based on current state; need external synchronization.
-func (s *session) pickCompaction() *compaction {
-	v := s.version()
-
-	var level int
-	var t0 tFiles
-	if v.cScore >= 1 {
-		level = v.cLevel
-		cptr := s.stCompPtrs[level]
-		tables := v.tables[level]
-		for _, t := range tables {
-			if cptr == nil || s.icmp.Compare(t.imax, cptr) > 0 {
-				t0 = append(t0, t)
-				break
-			}
-		}
-		if len(t0) == 0 {
-			t0 = append(t0, tables[0])
-		}
-	} else {
-		if p := atomic.LoadPointer(&v.cSeek); p != nil {
-			ts := (*tSet)(p)
-			level = ts.level
-			t0 = append(t0, ts.table)
-		} else {
-			v.release()
-			return nil
-		}
-	}
-
-	return newCompaction(s, v, level, t0)
-}
-
-// Create compaction from given level and range; need external synchronization.
-func (s *session) getCompactionRange(level int, umin, umax []byte) *compaction {
-	v := s.version()
-
-	t0 := v.tables[level].getOverlaps(nil, s.icmp, umin, umax, level == 0)
-	if len(t0) == 0 {
-		v.release()
-		return nil
-	}
-
-	// Avoid compacting too much in one shot in case the range is large.
-	// But we cannot do this for level-0 since level-0 files can overlap
-	// and we must not pick one file and drop another older file if the
-	// two files overlap.
-	if level > 0 {
-		limit := uint64(v.s.o.GetCompactionSourceLimit(level))
-		total := uint64(0)
-		for i, t := range t0 {
-			total += t.size
-			if total >= limit {
-				s.logf("table@compaction limiting F·%d -> F·%d", len(t0), i+1)
-				t0 = t0[:i+1]
-				break
-			}
-		}
-	}
-
-	return newCompaction(s, v, level, t0)
-}
-
-func newCompaction(s *session, v *version, level int, t0 tFiles) *compaction {
-	c := &compaction{
-		s:             s,
-		v:             v,
-		level:         level,
-		tables:        [2]tFiles{t0, nil},
-		maxGPOverlaps: uint64(s.o.GetCompactionGPOverlaps(level)),
-		tPtrs:         make([]int, s.o.GetNumLevel()),
-	}
-	c.expand()
-	c.save()
-	return c
-}
-
-// compaction represent a compaction state.
-type compaction struct {
-	s *session
-	v *version
-
-	level         int
-	tables        [2]tFiles
-	maxGPOverlaps uint64
-
-	gp                tFiles
-	gpi               int
-	seenKey           bool
-	gpOverlappedBytes uint64
-	imin, imax        iKey
-	tPtrs             []int
-	released          bool
-
-	snapGPI               int
-	snapSeenKey           bool
-	snapGPOverlappedBytes uint64
-	snapTPtrs             []int
-}
-
-func (c *compaction) save() {
-	c.snapGPI = c.gpi
-	c.snapSeenKey = c.seenKey
-	c.snapGPOverlappedBytes = c.gpOverlappedBytes
-	c.snapTPtrs = append(c.snapTPtrs[:0], c.tPtrs...)
-}
-
-func (c *compaction) restore() {
-	c.gpi = c.snapGPI
-	c.seenKey = c.snapSeenKey
-	c.gpOverlappedBytes = c.snapGPOverlappedBytes
-	c.tPtrs = append(c.tPtrs[:0], c.snapTPtrs...)
-}
-
-func (c *compaction) release() {
-	if !c.released {
-		c.released = true
-		c.v.release()
-	}
-}
-
-// Expand compacted tables; need external synchronization.
-func (c *compaction) expand() {
-	limit := uint64(c.s.o.GetCompactionExpandLimit(c.level))
-	vt0, vt1 := c.v.tables[c.level], c.v.tables[c.level+1]
-
-	t0, t1 := c.tables[0], c.tables[1]
-	imin, imax := t0.getRange(c.s.icmp)
-	// We expand t0 here just incase ukey hop across tables.
-	t0 = vt0.getOverlaps(t0, c.s.icmp, imin.ukey(), imax.ukey(), c.level == 0)
-	if len(t0) != len(c.tables[0]) {
-		imin, imax = t0.getRange(c.s.icmp)
-	}
-	t1 = vt1.getOverlaps(t1, c.s.icmp, imin.ukey(), imax.ukey(), false)
-	// Get entire range covered by compaction.
-	amin, amax := append(t0, t1...).getRange(c.s.icmp)
-
-	// See if we can grow the number of inputs in "level" without
-	// changing the number of "level+1" files we pick up.
-	if len(t1) > 0 {
-		exp0 := vt0.getOverlaps(nil, c.s.icmp, amin.ukey(), amax.ukey(), c.level == 0)
-		if len(exp0) > len(t0) && t1.size()+exp0.size() < limit {
-			xmin, xmax := exp0.getRange(c.s.icmp)
-			exp1 := vt1.getOverlaps(nil, c.s.icmp, xmin.ukey(), xmax.ukey(), false)
-			if len(exp1) == len(t1) {
-				c.s.logf("table@compaction expanding L%d+L%d (F·%d S·%s)+(F·%d S·%s) -> (F·%d S·%s)+(F·%d S·%s)",
-					c.level, c.level+1, len(t0), shortenb(int(t0.size())), len(t1), shortenb(int(t1.size())),
-					len(exp0), shortenb(int(exp0.size())), len(exp1), shortenb(int(exp1.size())))
-				imin, imax = xmin, xmax
-				t0, t1 = exp0, exp1
-				amin, amax = append(t0, t1...).getRange(c.s.icmp)
-			}
-		}
-	}
-
-	// Compute the set of grandparent files that overlap this compaction
-	// (parent == level+1; grandparent == level+2)
-	if c.level+2 < c.s.o.GetNumLevel() {
-		c.gp = c.v.tables[c.level+2].getOverlaps(c.gp, c.s.icmp, amin.ukey(), amax.ukey(), false)
-	}
-
-	c.tables[0], c.tables[1] = t0, t1
-	c.imin, c.imax = imin, imax
-}
-
-// Check whether compaction is trivial.
-func (c *compaction) trivial() bool {
-	return len(c.tables[0]) == 1 && len(c.tables[1]) == 0 && c.gp.size() <= c.maxGPOverlaps
-}
-
-func (c *compaction) baseLevelForKey(ukey []byte) bool {
-	for level, tables := range c.v.tables[c.level+2:] {
-		for c.tPtrs[level] < len(tables) {
-			t := tables[c.tPtrs[level]]
-			if c.s.icmp.uCompare(ukey, t.imax.ukey()) <= 0 {
-				// We've advanced far enough.
-				if c.s.icmp.uCompare(ukey, t.imin.ukey()) >= 0 {
-					// Key falls in this file's range, so definitely not base level.
-					return false
-				}
-				break
-			}
-			c.tPtrs[level]++
-		}
-	}
-	return true
-}
-
-func (c *compaction) shouldStopBefore(ikey iKey) bool {
-	for ; c.gpi < len(c.gp); c.gpi++ {
-		gp := c.gp[c.gpi]
-		if c.s.icmp.Compare(ikey, gp.imax) <= 0 {
-			break
-		}
-		if c.seenKey {
-			c.gpOverlappedBytes += gp.size
-		}
-	}
-	c.seenKey = true
-
-	if c.gpOverlappedBytes > c.maxGPOverlaps {
-		// Too much overlap for current output; start new output.
-		c.gpOverlappedBytes = 0
-		return true
-	}
-	return false
-}
-
-// Creates an iterator.
-func (c *compaction) newIterator() iterator.Iterator {
-	// Creates iterator slice.
-	icap := len(c.tables)
-	if c.level == 0 {
-		// Special case for level-0
-		icap = len(c.tables[0]) + 1
-	}
-	its := make([]iterator.Iterator, 0, icap)
-
-	// Options.
-	ro := &opt.ReadOptions{
-		DontFillCache: true,
-		Strict:        opt.StrictOverride,
-	}
-	strict := c.s.o.GetStrict(opt.StrictCompaction)
-	if strict {
-		ro.Strict |= opt.StrictReader
-	}
-
-	for i, tables := range c.tables {
-		if len(tables) == 0 {
-			continue
-		}
-
-		// Level-0 is not sorted and may overlaps each other.
-		if c.level+i == 0 {
-			for _, t := range tables {
-				its = append(its, c.s.tops.newIterator(t, nil, ro))
-			}
-		} else {
-			it := iterator.NewIndexedIterator(tables.newIndexIterator(c.s.tops, c.s.icmp, nil, ro), strict)
-			its = append(its, it)
-		}
-	}
-
-	return iterator.NewMergedIterator(its, c.s.icmp, strict)
-}
diff --git a/leveldb/session_compaction.go b/leveldb/session_compaction.go
new file mode 100644
index 0000000..7c5a794
--- /dev/null
+++ b/leveldb/session_compaction.go
@@ -0,0 +1,287 @@
+// Copyright (c) 2012, Suryandaru Triandana <syndtr@gmail.com>
+// All rights reserved.
+//
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package leveldb
+
+import (
+	"sync/atomic"
+
+	"github.com/syndtr/goleveldb/leveldb/iterator"
+	"github.com/syndtr/goleveldb/leveldb/memdb"
+	"github.com/syndtr/goleveldb/leveldb/opt"
+)
+
+func (s *session) pickMemdbLevel(umin, umax []byte) int {
+	v := s.version()
+	defer v.release()
+	return v.pickMemdbLevel(umin, umax)
+}
+
+func (s *session) flushMemdb(rec *sessionRecord, mdb *memdb.DB, level int) (level_ int, err error) {
+	// Create sorted table.
+	iter := mdb.NewIterator(nil)
+	defer iter.Release()
+	t, n, err := s.tops.createFrom(iter)
+	if err != nil {
+		return level, err
+	}
+
+	// Pick level and add to record.
+	if level < 0 {
+		level = s.pickMemdbLevel(t.imin.ukey(), t.imax.ukey())
+	}
+	rec.addTableFile(level, t)
+
+	s.logf("memdb@flush created L%d@%d N·%d S·%s %q:%q", level, t.file.Num(), n, shortenb(int(t.size)), t.imin, t.imax)
+	return level, nil
+}
+
+// Pick a compaction based on current state; need external synchronization.
+func (s *session) pickCompaction() *compaction {
+	v := s.version()
+
+	var level int
+	var t0 tFiles
+	if v.cScore >= 1 {
+		level = v.cLevel
+		cptr := s.stCompPtrs[level]
+		tables := v.tables[level]
+		for _, t := range tables {
+			if cptr == nil || s.icmp.Compare(t.imax, cptr) > 0 {
+				t0 = append(t0, t)
+				break
+			}
+		}
+		if len(t0) == 0 {
+			t0 = append(t0, tables[0])
+		}
+	} else {
+		if p := atomic.LoadPointer(&v.cSeek); p != nil {
+			ts := (*tSet)(p)
+			level = ts.level
+			t0 = append(t0, ts.table)
+		} else {
+			v.release()
+			return nil
+		}
+	}
+
+	return newCompaction(s, v, level, t0)
+}
+
+// Create compaction from given level and range; need external synchronization.
+func (s *session) getCompactionRange(level int, umin, umax []byte) *compaction {
+	v := s.version()
+
+	t0 := v.tables[level].getOverlaps(nil, s.icmp, umin, umax, level == 0)
+	if len(t0) == 0 {
+		v.release()
+		return nil
+	}
+
+	// Avoid compacting too much in one shot in case the range is large.
+	// But we cannot do this for level-0 since level-0 files can overlap
+	// and we must not pick one file and drop another older file if the
+	// two files overlap.
+	if level > 0 {
+		limit := uint64(v.s.o.GetCompactionSourceLimit(level))
+		total := uint64(0)
+		for i, t := range t0 {
+			total += t.size
+			if total >= limit {
+				s.logf("table@compaction limiting F·%d -> F·%d", len(t0), i+1)
+				t0 = t0[:i+1]
+				break
+			}
+		}
+	}
+
+	return newCompaction(s, v, level, t0)
+}
+
+func newCompaction(s *session, v *version, level int, t0 tFiles) *compaction {
+	c := &compaction{
+		s:             s,
+		v:             v,
+		level:         level,
+		tables:        [2]tFiles{t0, nil},
+		maxGPOverlaps: uint64(s.o.GetCompactionGPOverlaps(level)),
+		tPtrs:         make([]int, s.o.GetNumLevel()),
+	}
+	c.expand()
+	c.save()
+	return c
+}
+
+// compaction represent a compaction state.
+type compaction struct {
+	s *session
+	v *version
+
+	level         int
+	tables        [2]tFiles
+	maxGPOverlaps uint64
+
+	gp                tFiles
+	gpi               int
+	seenKey           bool
+	gpOverlappedBytes uint64
+	imin, imax        iKey
+	tPtrs             []int
+	released          bool
+
+	snapGPI               int
+	snapSeenKey           bool
+	snapGPOverlappedBytes uint64
+	snapTPtrs             []int
+}
+
+func (c *compaction) save() {
+	c.snapGPI = c.gpi
+	c.snapSeenKey = c.seenKey
+	c.snapGPOverlappedBytes = c.gpOverlappedBytes
+	c.snapTPtrs = append(c.snapTPtrs[:0], c.tPtrs...)
+}
+
+func (c *compaction) restore() {
+	c.gpi = c.snapGPI
+	c.seenKey = c.snapSeenKey
+	c.gpOverlappedBytes = c.snapGPOverlappedBytes
+	c.tPtrs = append(c.tPtrs[:0], c.snapTPtrs...)
+}
+
+func (c *compaction) release() {
+	if !c.released {
+		c.released = true
+		c.v.release()
+	}
+}
+
+// Expand compacted tables; need external synchronization.
+func (c *compaction) expand() {
+	limit := uint64(c.s.o.GetCompactionExpandLimit(c.level))
+	vt0, vt1 := c.v.tables[c.level], c.v.tables[c.level+1]
+
+	t0, t1 := c.tables[0], c.tables[1]
+	imin, imax := t0.getRange(c.s.icmp)
+	// We expand t0 here just incase ukey hop across tables.
+	t0 = vt0.getOverlaps(t0, c.s.icmp, imin.ukey(), imax.ukey(), c.level == 0)
+	if len(t0) != len(c.tables[0]) {
+		imin, imax = t0.getRange(c.s.icmp)
+	}
+	t1 = vt1.getOverlaps(t1, c.s.icmp, imin.ukey(), imax.ukey(), false)
+	// Get entire range covered by compaction.
+	amin, amax := append(t0, t1...).getRange(c.s.icmp)
+
+	// See if we can grow the number of inputs in "level" without
+	// changing the number of "level+1" files we pick up.
+	if len(t1) > 0 {
+		exp0 := vt0.getOverlaps(nil, c.s.icmp, amin.ukey(), amax.ukey(), c.level == 0)
+		if len(exp0) > len(t0) && t1.size()+exp0.size() < limit {
+			xmin, xmax := exp0.getRange(c.s.icmp)
+			exp1 := vt1.getOverlaps(nil, c.s.icmp, xmin.ukey(), xmax.ukey(), false)
+			if len(exp1) == len(t1) {
+				c.s.logf("table@compaction expanding L%d+L%d (F·%d S·%s)+(F·%d S·%s) -> (F·%d S·%s)+(F·%d S·%s)",
+					c.level, c.level+1, len(t0), shortenb(int(t0.size())), len(t1), shortenb(int(t1.size())),
+					len(exp0), shortenb(int(exp0.size())), len(exp1), shortenb(int(exp1.size())))
+				imin, imax = xmin, xmax
+				t0, t1 = exp0, exp1
+				amin, amax = append(t0, t1...).getRange(c.s.icmp)
+			}
+		}
+	}
+
+	// Compute the set of grandparent files that overlap this compaction
+	// (parent == level+1; grandparent == level+2)
+	if c.level+2 < c.s.o.GetNumLevel() {
+		c.gp = c.v.tables[c.level+2].getOverlaps(c.gp, c.s.icmp, amin.ukey(), amax.ukey(), false)
+	}
+
+	c.tables[0], c.tables[1] = t0, t1
+	c.imin, c.imax = imin, imax
+}
+
+// Check whether compaction is trivial.
+func (c *compaction) trivial() bool {
+	return len(c.tables[0]) == 1 && len(c.tables[1]) == 0 && c.gp.size() <= c.maxGPOverlaps
+}
+
+func (c *compaction) baseLevelForKey(ukey []byte) bool {
+	for level, tables := range c.v.tables[c.level+2:] {
+		for c.tPtrs[level] < len(tables) {
+			t := tables[c.tPtrs[level]]
+			if c.s.icmp.uCompare(ukey, t.imax.ukey()) <= 0 {
+				// We've advanced far enough.
+				if c.s.icmp.uCompare(ukey, t.imin.ukey()) >= 0 {
+					// Key falls in this file's range, so definitely not base level.
+					return false
+				}
+				break
+			}
+			c.tPtrs[level]++
+		}
+	}
+	return true
+}
+
+func (c *compaction) shouldStopBefore(ikey iKey) bool {
+	for ; c.gpi < len(c.gp); c.gpi++ {
+		gp := c.gp[c.gpi]
+		if c.s.icmp.Compare(ikey, gp.imax) <= 0 {
+			break
+		}
+		if c.seenKey {
+			c.gpOverlappedBytes += gp.size
+		}
+	}
+	c.seenKey = true
+
+	if c.gpOverlappedBytes > c.maxGPOverlaps {
+		// Too much overlap for current output; start new output.
+		c.gpOverlappedBytes = 0
+		return true
+	}
+	return false
+}
+
+// Creates an iterator.
+func (c *compaction) newIterator() iterator.Iterator {
+	// Creates iterator slice.
+	icap := len(c.tables)
+	if c.level == 0 {
+		// Special case for level-0.
+		icap = len(c.tables[0]) + 1
+	}
+	its := make([]iterator.Iterator, 0, icap)
+
+	// Options.
+	ro := &opt.ReadOptions{
+		DontFillCache: true,
+		Strict:        opt.StrictOverride,
+	}
+	strict := c.s.o.GetStrict(opt.StrictCompaction)
+	if strict {
+		ro.Strict |= opt.StrictReader
+	}
+
+	for i, tables := range c.tables {
+		if len(tables) == 0 {
+			continue
+		}
+
+		// Level-0 is not sorted and may overlaps each other.
+		if c.level+i == 0 {
+			for _, t := range tables {
+				its = append(its, c.s.tops.newIterator(t, nil, ro))
+			}
+		} else {
+			it := iterator.NewIndexedIterator(tables.newIndexIterator(c.s.tops, c.s.icmp, nil, ro), strict)
+			its = append(its, it)
+		}
+	}
+
+	return iterator.NewMergedIterator(its, c.s.icmp, strict)
+}
diff --git a/leveldb/session_record.go b/leveldb/session_record.go
index 1bdcc68..405e07b 100644
--- a/leveldb/session_record.go
+++ b/leveldb/session_record.go
@@ -52,8 +52,6 @@
 }
 
 type sessionRecord struct {
-	numLevel int
-
 	hasRec         int
 	comparer       string
 	journalNum     uint64
@@ -230,7 +228,7 @@
 	return x
 }
 
-func (p *sessionRecord) readLevel(field string, r io.ByteReader) int {
+func (p *sessionRecord) readLevel(field string, r io.ByteReader, numLevel int) int {
 	if p.err != nil {
 		return 0
 	}
@@ -238,14 +236,14 @@
 	if p.err != nil {
 		return 0
 	}
-	if x >= uint64(p.numLevel) {
+	if x >= uint64(numLevel) {
 		p.err = errors.NewErrCorrupted(nil, &ErrManifestCorrupted{field, "invalid level number"})
 		return 0
 	}
 	return int(x)
 }
 
-func (p *sessionRecord) decode(r io.Reader) error {
+func (p *sessionRecord) decode(r io.Reader, numLevel int) error {
 	br, ok := r.(byteReader)
 	if !ok {
 		br = bufio.NewReader(r)
@@ -286,13 +284,13 @@
 				p.setSeqNum(x)
 			}
 		case recCompPtr:
-			level := p.readLevel("comp-ptr.level", br)
+			level := p.readLevel("comp-ptr.level", br, numLevel)
 			ikey := p.readBytes("comp-ptr.ikey", br)
 			if p.err == nil {
 				p.addCompPtr(level, iKey(ikey))
 			}
 		case recAddTable:
-			level := p.readLevel("add-table.level", br)
+			level := p.readLevel("add-table.level", br, numLevel)
 			num := p.readUvarint("add-table.num", br)
 			size := p.readUvarint("add-table.size", br)
 			imin := p.readBytes("add-table.imin", br)
@@ -301,7 +299,7 @@
 				p.addTable(level, num, size, imin, imax)
 			}
 		case recDelTable:
-			level := p.readLevel("del-table.level", br)
+			level := p.readLevel("del-table.level", br, numLevel)
 			num := p.readUvarint("del-table.num", br)
 			if p.err == nil {
 				p.delTable(level, num)
diff --git a/leveldb/session_record_test.go b/leveldb/session_record_test.go
index c0c035a..33c1487 100644
--- a/leveldb/session_record_test.go
+++ b/leveldb/session_record_test.go
@@ -19,8 +19,8 @@
 	if err != nil {
 		return
 	}
-	v2 := &sessionRecord{numLevel: opt.DefaultNumLevel}
-	err = v.decode(b)
+	v2 := &sessionRecord{}
+	err = v.decode(b, opt.DefaultNumLevel)
 	if err != nil {
 		return
 	}
@@ -34,7 +34,7 @@
 
 func TestSessionRecord_EncodeDecode(t *testing.T) {
 	big := uint64(1) << 50
-	v := &sessionRecord{numLevel: opt.DefaultNumLevel}
+	v := &sessionRecord{}
 	i := uint64(0)
 	test := func() {
 		res, err := decodeEncode(v)
diff --git a/leveldb/session_util.go b/leveldb/session_util.go
index 007c02c..399a788 100644
--- a/leveldb/session_util.go
+++ b/leveldb/session_util.go
@@ -182,7 +182,7 @@
 		defer v.release()
 	}
 	if rec == nil {
-		rec = &sessionRecord{numLevel: s.o.GetNumLevel()}
+		rec = &sessionRecord{}
 	}
 	s.fillRecord(rec, true)
 	v.fillRecord(rec)
diff --git a/leveldb/version.go b/leveldb/version.go
index 88a52f5..011d982 100644
--- a/leveldb/version.go
+++ b/leveldb/version.go
@@ -300,7 +300,7 @@
 	return
 }
 
-func (v *version) pickLevel(umin, umax []byte) (level int) {
+func (v *version) pickMemdbLevel(umin, umax []byte) (level int) {
 	if !v.tables[0].overlaps(v.s.icmp, umin, umax, true) {
 		var overlaps tFiles
 		maxLevel := v.s.o.GetMaxMemCompationLevel()