fix: reduce virtual address space usage to support 32-bit (386) architectures

db.go

@@ -606,7 +606,8 @@ func (db *DB) close() (err error) {
 					select {
 					case db.flushChan <- db.mt:
 						db.imm = append(db.imm, db.mt) // Flusher will attempt to remove this from s.imm.
-						db.mt = nil                    // Will segfault if we try writing!
+						db.mt.releaseWAL()
+						db.mt = nil // Will segfault if we try writing!
 						db.opt.Debugf("pushed to flush chan\n")
 						return true
 					default:
@@ -1058,6 +1059,7 @@ func (db *DB) ensureRoomForWrite() error {
 				db.mt.sl.MemSize(), len(db.flushChan))
 			// We manage to push this task. Let's modify imm.
 			db.imm = append(db.imm, db.mt)
+			db.mt.releaseWAL()
 			db.mt, err = db.newMemTable()
 			if err != nil {
 				return y.Wrapf(err, "cannot create new mem table")
@@ -1882,6 +1884,7 @@ func (db *DB) DropPrefix(prefixes ...[]byte) error {
 	defer db.lock.Unlock()
 
 	db.imm = append(db.imm, db.mt)
+	db.mt.releaseWAL()
 	for _, memtable := range db.imm {
 		if memtable.sl.Empty() {
 			memtable.DecrRef()

memtable.go

@@ -84,6 +84,8 @@ func (db *DB) openMemTables(opt Options) error {
 		}
 		// These should no longer be written to. So, make them part of the imm.
 		db.imm = append(db.imm, mt)
+		// WAL data has been replayed into the skiplist — release the mmap.
+		mt.releaseWAL()
 	}
 	if len(fids) != 0 {
 		db.nextMemFid = fids[len(fids)-1]
@@ -222,6 +224,28 @@ func (mt *memTable) DecrRef() {
 	mt.sl.DecrRef()
 }
 
+// releaseWAL unmaps the WAL mmap for an immutable memtable. After the WAL has been replayed
+// into the skiplist (via UpdateSkipList), the mmap'd region is no longer needed — all reads
+// go through the skiplist. On 32-bit systems this frees ~128 MB of virtual address space per
+// memtable, which is critical given the ~3 GB user-space limit.
+func (mt *memTable) releaseWAL() {
+	if mt.wal == nil || mt.wal.Fd == nil {
+		return
+	}
+	path := mt.wal.Fd.Name()
+	if mt.wal.Data != nil {
+		_ = z.Munmap(mt.wal.Data)
+		mt.wal.Data = nil
+	}
+	_ = mt.wal.Fd.Close()
+	mt.wal.Fd = nil
+	// Replace the OnClose callback: we've already unmapped the data and
+	// closed the fd, so just remove the file when the skiplist is freed.
+	mt.sl.OnClose = func() {
+		_ = os.Remove(path)
+	}
+}
+
 func (mt *memTable) replayFunction(opt Options) func(Entry, valuePointer) error {
 	first := true
 	return func(e Entry, _ valuePointer) error { // Function for replaying.