tsdb: create isolation transaction slice on demand

When Prometheus restarts it creates every series read in from the WAL,
but many of those series will be finished, and never receive any more
samples. By defering allocation of the txRing slice to when it is first
needed, we save 32 bytes per stale series.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/head.go

@@ -2011,7 +2011,7 @@ func newMemSeries(lset labels.Labels, id chunks.HeadSeriesRef, isolationDisabled
 		nextAt: math.MinInt64,
 	}
 	if !isolationDisabled {
-		s.txs = newTxRing(4)
+		s.txs = newTxRing(0)
 	}
 	return s
 }

tsdb/isolation.go

@@ -253,7 +253,11 @@ func newTxRing(capacity int) *txRing {
 func (txr *txRing) add(appendID uint64) {
 	if int(txr.txIDCount) == len(txr.txIDs) {
 		// Ring buffer is full, expand by doubling.
-		newRing := make([]uint64, txr.txIDCount*2)
+		newLen := txr.txIDCount * 2
+		if newLen == 0 {
+			newLen = 4
+		}
+		newRing := make([]uint64, newLen)
 		idx := copy(newRing, txr.txIDs[txr.txIDFirst:])
 		copy(newRing[idx:], txr.txIDs[:txr.txIDFirst])
 		txr.txIDs = newRing
@@ -265,6 +269,9 @@ func (txr *txRing) add(appendID uint64) {
 }
 
 func (txr *txRing) cleanupAppendIDsBelow(bound uint64) {
+	if len(txr.txIDs) == 0 {
+		return
+	}
 	pos := int(txr.txIDFirst)
 
 	for txr.txIDCount > 0 {