Reduce allocation in wal encoder

Use sync.Pool for some temporary buffers used while encoding instead of
allocatin new ones each time.  Also increased the default buffer size which
might be too small.  Probably need to make this a config var.

tsdb/engine/tsm1/wal.go

@@ -27,6 +27,9 @@ const (
 
 	defaultBufLen = 1024 << 10 // 1MB (sized for batches of 5000 points)
 
+	// walEncodeBufSize is the size of the wal entry encoding buffer
+	walEncodeBufSize = 4 * 1024 * 1024
+
 	float64EntryType = 1
 	int64EntryType   = 2
 	boolEntryType    = 3
@@ -203,14 +206,17 @@ func (l *WAL) LastWriteTime() time.Time {
 
 func (l *WAL) writeToLog(entry WALEntry) (int, error) {
 	// encode and compress the entry while we're not locked
-	bytes := make([]byte, defaultBufLen)
+	bytes := getBuf(walEncodeBufSize)
+	defer putBuf(bytes)
 
 	b, err := entry.Encode(bytes)
 	if err != nil {
 		return -1, err
 	}
 
-	compressed := snappy.Encode(b, b)
+	encBuf := getBuf(snappy.MaxEncodedLen(len(b)))
+	defer putBuf(encBuf)
+	compressed := snappy.Encode(encBuf, b)
 
 	l.mu.Lock()
 	defer l.mu.Unlock()
@@ -387,7 +393,7 @@ func (w *WriteWALEntry) Encode(dst []byte) ([]byte, error) {
 		n++
 
 		n += copy(dst[n:], u16tob(uint16(len(k))))
-		n += copy(dst[n:], []byte(k))
+		n += copy(dst[n:], k)
 
 		n += copy(dst[n:], u32tob(uint32(len(v))))
 
@@ -642,7 +648,15 @@ func (r *WALSegmentReader) Next() bool {
 	}
 	nReadOK += n
 
-	data, err := snappy.Decode(nil, b[:length])
+	decLen, err := snappy.DecodedLen(b[:length])
+	if err != nil {
+		r.err = err
+		return true
+	}
+	decBuf := getBuf(decLen)
+	defer putBuf(decBuf)
+
+	data, err := snappy.Decode(decBuf, b[:length])
 	if err != nil {
 		r.err = err
 		return true