sstable: add separated value retrieval internal iterator stats

Add internal iterator stats for the count of separated value retrievals and the
subset of them that miss the iterator's reader cache.

Informs cockroachdb/cockroach#149422.

Author: jbowens

internal/base/iterator.go

@@ -432,6 +432,11 @@ type InternalIteratorStats struct {
 		// in the heap got positioned. The count here includes every sstable
 		// iterator that got positioned in the heap.
 		Count uint64
+		// CountFetched is the subset of Count that were fetched.
+		CountFetched uint64
+		// ReaderCacheMisses is the count of separated value retrievals that did
+		// not find the named blob file reader in the iterator's reader cache.
+		ReaderCacheMisses uint64
 		// ValueBytes represent the total byte length of the values (in value
 		// blocks) of the points corresponding to Count.
 		ValueBytes uint64
@@ -454,6 +459,8 @@ func (s *InternalIteratorStats) Merge(from InternalIteratorStats) {
 	s.PointCount += from.PointCount
 	s.PointsCoveredByRangeTombstones += from.PointsCoveredByRangeTombstones
 	s.SeparatedPointValue.Count += from.SeparatedPointValue.Count
+	s.SeparatedPointValue.CountFetched += from.SeparatedPointValue.CountFetched
+	s.SeparatedPointValue.ReaderCacheMisses += from.SeparatedPointValue.ReaderCacheMisses
 	s.SeparatedPointValue.ValueBytes += from.SeparatedPointValue.ValueBytes
 	s.SeparatedPointValue.ValueBytesFetched += from.SeparatedPointValue.ValueBytesFetched
 }

sstable/blob/fetcher.go

@@ -164,9 +164,13 @@ func (r *ValueFetcher) retrieve(ctx context.Context, vh Handle) (val []byte, err
 		cr.blobFileID = vh.BlobFileID
 		cr.diskFileNum = diskFileNum
 		cr.rh = cr.r.InitReadHandle(&cr.preallocRH)
+		if r.env.Stats != nil {
+			r.env.Stats.SeparatedPointValue.ReaderCacheMisses++
+		}
 	}
 
 	if r.env.Stats != nil {
+		r.env.Stats.SeparatedPointValue.CountFetched++
 		r.env.Stats.SeparatedPointValue.ValueBytesFetched += uint64(vh.ValueLen)
 	}
 

sstable/valblk/reader.go

@@ -261,6 +261,7 @@ func (f *valueBlockFetcher) getValueInternal(handle []byte, valLen uint32) (val
 		f.valueBlockPtr = unsafe.Pointer(&f.valueBlock[0])
 	}
 	if f.stats != nil {
+		f.stats.SeparatedPointValue.CountFetched++
 		f.stats.SeparatedPointValue.ValueBytesFetched += uint64(valLen)
 	}
 	return f.valueBlock[vh.OffsetInBlock : vh.OffsetInBlock+vh.ValueLen], nil