From 29e4e7f4229fc7b7660a6d88fb37aa9a1609b534 Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@gmail.com>
Date: Sat, 31 Oct 2020 20:42:13 +0200
Subject: [PATCH] lib/storage: drop more samples outside the given retention
 during background merge

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/17
---
 lib/storage/block.go |  2 +-
 lib/storage/merge.go | 65 ++++++++++++++++++++++----------------------
 2 files changed, 33 insertions(+), 34 deletions(-)

diff --git a/lib/storage/block.go b/lib/storage/block.go
index ec0b3fdeb..c446c370e 100644
--- a/lib/storage/block.go
+++ b/lib/storage/block.go
@@ -23,7 +23,7 @@ const (
 type Block struct {
 	bh blockHeader
 
-	// nextIdx is the next row index for timestamps and values.
+	// nextIdx is the next index for reading timestamps and values.
 	nextIdx int
 
 	timestamps []int64
diff --git a/lib/storage/merge.go b/lib/storage/merge.go
index 6a1845ce9..0fc16d434 100644
--- a/lib/storage/merge.go
+++ b/lib/storage/merge.go
@@ -40,8 +40,11 @@ var errForciblyStopped = fmt.Errorf("forcibly stopped")
 
 func mergeBlockStreamsInternal(ph *partHeader, bsw *blockStreamWriter, bsm *blockStreamMerger, stopCh <-chan struct{},
 	dmis *uint64set.Set, retentionDeadline int64, rowsMerged, rowsDeleted *uint64) error {
-	// Search for the first block to merge
-	var pendingBlock *Block
+	pendingBlockIsEmpty := true
+	pendingBlock := getBlock()
+	defer putBlock(pendingBlock)
+	tmpBlock := getBlock()
+	defer putBlock(tmpBlock)
 	for bsm.NextBlock() {
 		select {
 		case <-stopCh:
@@ -58,31 +61,10 @@ func mergeBlockStreamsInternal(ph *partHeader, bsw *blockStreamWriter, bsm *bloc
 			*rowsDeleted += uint64(bsm.Block.bh.RowsCount)
 			continue
 		}
-		pendingBlock = getBlock()
-		pendingBlock.CopyFrom(bsm.Block)
-		break
-	}
-	if pendingBlock != nil {
-		defer putBlock(pendingBlock)
-	}
-
-	// Merge blocks.
-	tmpBlock := getBlock()
-	defer putBlock(tmpBlock)
-	for bsm.NextBlock() {
-		select {
-		case <-stopCh:
-			return errForciblyStopped
-		default:
-		}
-		if dmis.Has(bsm.Block.bh.TSID.MetricID) {
-			// Skip blocks for deleted metrics.
-			*rowsDeleted += uint64(bsm.Block.bh.RowsCount)
-			continue
-		}
-		if bsm.Block.bh.MaxTimestamp < retentionDeadline {
-			// skip blocks out of the given retention.
-			*rowsDeleted += uint64(bsm.Block.bh.RowsCount)
+		if pendingBlockIsEmpty {
+			// Load the next block if pendingBlock is empty.
+			pendingBlock.CopyFrom(bsm.Block)
+			pendingBlockIsEmpty = false
 			continue
 		}
 
@@ -114,16 +96,20 @@ func mergeBlockStreamsInternal(ph *partHeader, bsw *blockStreamWriter, bsm *bloc
 		tmpBlock.bh.TSID = bsm.Block.bh.TSID
 		tmpBlock.bh.Scale = bsm.Block.bh.Scale
 		tmpBlock.bh.PrecisionBits = minUint8(pendingBlock.bh.PrecisionBits, bsm.Block.bh.PrecisionBits)
-		mergeBlocks(tmpBlock, pendingBlock, bsm.Block)
+		mergeBlocks(tmpBlock, pendingBlock, bsm.Block, retentionDeadline, rowsDeleted)
 		if len(tmpBlock.timestamps) <= maxRowsPerBlock {
 			// More entries may be added to tmpBlock. Swap it with pendingBlock,
 			// so more entries may be added to pendingBlock on the next iteration.
-			tmpBlock.fixupTimestamps()
+			if len(tmpBlock.timestamps) > 0 {
+				tmpBlock.fixupTimestamps()
+			} else {
+				pendingBlockIsEmpty = true
+			}
 			pendingBlock, tmpBlock = tmpBlock, pendingBlock
 			continue
 		}
 
-		// Write the first len(maxRowsPerBlock) of tmpBlock.timestamps to bsw,
+		// Write the first maxRowsPerBlock of tmpBlock.timestamps to bsw,
 		// leave the rest in pendingBlock.
 		tmpBlock.nextIdx = maxRowsPerBlock
 		pendingBlock.CopyFrom(tmpBlock)
@@ -137,18 +123,21 @@ func mergeBlockStreamsInternal(ph *partHeader, bsw *blockStreamWriter, bsm *bloc
 	if err := bsm.Error(); err != nil {
 		return fmt.Errorf("cannot read block to be merged: %w", err)
 	}
-	if pendingBlock != nil {
+	if !pendingBlockIsEmpty {
 		bsw.WriteExternalBlock(pendingBlock, ph, rowsMerged)
 	}
 	return nil
 }
 
 // mergeBlocks merges ib1 and ib2 to ob.
-func mergeBlocks(ob, ib1, ib2 *Block) {
+func mergeBlocks(ob, ib1, ib2 *Block, retentionDeadline int64, rowsDeleted *uint64) {
 	ib1.assertMergeable(ib2)
 	ib1.assertUnmarshaled()
 	ib2.assertUnmarshaled()
 
+	skipSamplesOutsideRetention(ib1, retentionDeadline, rowsDeleted)
+	skipSamplesOutsideRetention(ib2, retentionDeadline, rowsDeleted)
+
 	if ib1.bh.MaxTimestamp < ib2.bh.MinTimestamp {
 		// Fast path - ib1 values have smaller timestamps than ib2 values.
 		appendRows(ob, ib1)
@@ -186,6 +175,16 @@ func mergeBlocks(ob, ib1, ib2 *Block) {
 	}
 }
 
+func skipSamplesOutsideRetention(b *Block, retentionDeadline int64, rowsDeleted *uint64) {
+	timestamps := b.timestamps
+	nextIdx := b.nextIdx
+	for nextIdx < len(timestamps) && timestamps[nextIdx] < retentionDeadline {
+		nextIdx++
+	}
+	*rowsDeleted += uint64(nextIdx - b.nextIdx)
+	b.nextIdx = nextIdx
+}
+
 func appendRows(ob, ib *Block) {
 	ob.timestamps = append(ob.timestamps, ib.timestamps[ib.nextIdx:]...)
 	ob.values = append(ob.values, ib.values[ib.nextIdx:]...)
@@ -199,7 +198,7 @@ func unmarshalAndCalibrateScale(b1, b2 *Block) error {
 		return err
 	}
 
-	scale := decimal.CalibrateScale(b1.values, b1.bh.Scale, b2.values, b2.bh.Scale)
+	scale := decimal.CalibrateScale(b1.values[b1.nextIdx:], b1.bh.Scale, b2.values[b2.nextIdx:], b2.bh.Scale)
 	b1.bh.Scale = scale
 	b2.bh.Scale = scale
 	return nil