pebble: Collect Pebble Key Statistics

Created a new function `ScanStatistics` that returns counts of the
different key kinds in Pebble (by level) as well as the number of
snapshot keys. Also modified `ScanInternal` to surface the level of each
key within each visitor function.

Informs: #1996

db.go

@@ -1178,21 +1178,28 @@ func finishInitializingIter(ctx context.Context, buf *iterAlloc) *Iterator {
 func (d *DB) ScanInternal(
 	ctx context.Context,
 	lower, upper []byte,
-	visitPointKey func(key *InternalKey, value LazyValue) error,
+	visitPointKey func(key *InternalKey, value LazyValue, iterInfo IteratorLevel) error,
 	visitRangeDel func(start, end []byte, seqNum uint64) error,
 	visitRangeKey func(start, end []byte, keys []rangekey.Key) error,
 	visitSharedFile func(sst *SharedSSTMeta) error,
+	includeObsoleteKeys bool,
 ) error {
-	iter := d.newInternalIter(nil /* snapshot */, &scanInternalOptions{
+	scanInternalOpts := &scanInternalOptions{
+		visitPointKey:       visitPointKey,
+		visitRangeDel:       visitRangeDel,
+		visitRangeKey:       visitRangeKey,
+		visitSharedFile:     visitSharedFile,
+		skipSharedLevels:    visitSharedFile != nil,
+		includeObsoleteKeys: includeObsoleteKeys,
 		IterOptions: IterOptions{
 			KeyTypes:   IterKeyTypePointsAndRanges,
 			LowerBound: lower,
 			UpperBound: upper,
 		},
-		skipSharedLevels: visitSharedFile != nil,
-	})
+	}
+	iter := d.newInternalIter(nil /* snapshot */, scanInternalOpts)
 	defer iter.close()
-	return scanInternalImpl(ctx, lower, upper, iter, visitPointKey, visitRangeDel, visitRangeKey, visitSharedFile)
+	return scanInternalImpl(ctx, lower, upper, iter, scanInternalOpts)
 }
 
 // newInternalIter constructs and returns a new scanInternalIterator on this db.
@@ -1231,6 +1238,7 @@ func (d *DB) newInternalIter(s *Snapshot, o *scanInternalOptions) *scanInternalI
 		newIters:        d.newIters,
 		newIterRangeKey: d.tableNewRangeKeyIter,
 		seqNum:          seqNum,
+		mergingIter:     &buf.merging,
 	}
 	if o != nil {
 		dbi.opts = *o
@@ -2021,7 +2029,6 @@ func (d *DB) SSTables(opts ...SSTablesOption) ([][]SSTableInfo, error) {
 			if opt.start != nil && opt.end != nil && !m.Overlaps(d.opts.Comparer.Compare, opt.start, opt.end, true /* exclusive end */) {
 				continue
 			}
-
 			destTables[j] = SSTableInfo{TableInfo: m.TableInfo()}
 			if opt.withProperties {
 				p, err := d.tableCache.getTableProperties(
@@ -2644,6 +2651,78 @@ func (d *DB) SetCreatorID(creatorID uint64) error {
 	return d.objProvider.SetCreatorID(objstorage.CreatorID(creatorID))
 }
 
+// KeyStatistics keeps track of the number of keys that have been pinned by a
+// snapshot as well as counts of the different key kinds in the lsm.
+type KeyStatistics struct {
+	// when a compaction determines a key is obsolete, but cannot elide the key
+	// because it's required by an open snapshot.
+	snapshotPinnedKeys int
+	// the total number of bytes of all snapshot pinned keys.
+	snapshotPinnedKeysBytes uint64
+	// Note: these fields are currently only populated for point keys (including range deletes).
+	kindsCount [InternalKeyKindMax + 1]int
+}
+
+// LSMKeyStatistics is used by DB.ScanStatistics.
+type LSMKeyStatistics struct {
+	accumulated KeyStatistics
+	levels      [numLevels]KeyStatistics
+	bytesRead   uint64
+}
+
+// ScanStatistics returns the count of different key kinds within the lsm for a
+// key span [lower, upper) as well as the number of snapshot keys.
+func (d *DB) ScanStatistics(ctx context.Context, lower, upper []byte) (LSMKeyStatistics, error) {
+	stats := LSMKeyStatistics{}
+	var prevKey InternalKey
+
+	err := d.ScanInternal(ctx, lower, upper,
+		func(key *InternalKey, value LazyValue, iterInfo IteratorLevel) error {
+			// If the previous key is equal to the current point key, the current key was
+			// pinned by a snapshot.
+			size := uint64(key.Size())
+			kind := key.Kind()
+			if iterInfo.kind != IteratorLevelFlushable && d.equal(prevKey.UserKey, key.UserKey) {
+				if iterInfo.kind == IteratorLevelLSM {
+					stats.levels[iterInfo.Level].snapshotPinnedKeys++
+					stats.levels[iterInfo.Level].snapshotPinnedKeysBytes += size
+				}
+				stats.accumulated.snapshotPinnedKeys++
+				stats.accumulated.snapshotPinnedKeysBytes += size
+			}
+			if iterInfo.kind == IteratorLevelLSM {
+				stats.levels[iterInfo.Level].kindsCount[kind]++
+			}
+
+			stats.accumulated.kindsCount[kind]++
+			prevKey.CopyFrom(*key)
+			stats.bytesRead += uint64(key.Size() + value.Len())
+			return nil
+		},
+		func(start, end []byte, seqNum uint64) error {
+			stats.accumulated.kindsCount[InternalKeyKindRangeDelete]++
+			stats.bytesRead += uint64(len(start) + len(end))
+			return nil
+		},
+		func(start, end []byte, keys []rangekey.Key) error {
+			stats.bytesRead += uint64(len(start) + len(end))
+			for _, key := range keys {
+				stats.accumulated.kindsCount[key.Kind()]++
+				stats.bytesRead += uint64(len(key.Value) + len(key.Suffix))
+			}
+			return nil
+		},
+		nil,
+		true,
+	)
+
+	if err != nil {
+		return LSMKeyStatistics{}, err
+	}
+
+	return stats, nil
+}
+
 // ObjProvider returns the objstorage.Provider for this database. Meant to be
 // used for internal purposes only.
 func (d *DB) ObjProvider() objstorage.Provider {

ingest_test.go

@@ -905,29 +905,34 @@ func TestIngestShared(t *testing.T) {
 			w := sstable.NewWriter(objstorageprovider.NewFileWritable(f), writeOpts)
 
 			var sharedSSTs []SharedSSTMeta
-			err = from.ScanInternal(context.TODO(), startKey, endKey, func(key *InternalKey, value LazyValue) error {
-				val, _, err := value.Value(nil)
-				require.NoError(t, err)
-				require.NoError(t, w.Add(base.MakeInternalKey(key.UserKey, 0, key.Kind()), val))
-				return nil
-			}, func(start, end []byte, seqNum uint64) error {
-				require.NoError(t, w.DeleteRange(start, end))
-				return nil
-			}, func(start, end []byte, keys []keyspan.Key) error {
-				s := keyspan.Span{
-					Start:     start,
-					End:       end,
-					Keys:      keys,
-					KeysOrder: 0,
-				}
-				require.NoError(t, rangekey.Encode(&s, func(k base.InternalKey, v []byte) error {
-					return w.AddRangeKey(base.MakeInternalKey(k.UserKey, 0, k.Kind()), v)
-				}))
-				return nil
-			}, func(sst *SharedSSTMeta) error {
-				sharedSSTs = append(sharedSSTs, *sst)
-				return nil
-			})
+			err = from.ScanInternal(context.TODO(), startKey, endKey,
+				func(key *InternalKey, value LazyValue, _ IteratorLevel) error {
+					val, _, err := value.Value(nil)
+					require.NoError(t, err)
+					require.NoError(t, w.Add(base.MakeInternalKey(key.UserKey, 0, key.Kind()), val))
+					return nil
+				},
+				func(start, end []byte, seqNum uint64) error {
+					require.NoError(t, w.DeleteRange(start, end))
+					return nil
+				},
+				func(start, end []byte, keys []keyspan.Key) error {
+					s := keyspan.Span{
+						Start:     start,
+						End:       end,
+						Keys:      keys,
+						KeysOrder: 0,
+					}
+					require.NoError(t, rangekey.Encode(&s, func(k base.InternalKey, v []byte) error {
+						return w.AddRangeKey(base.MakeInternalKey(k.UserKey, 0, k.Kind()), v)
+					}))
+					return nil
+				},
+				func(sst *SharedSSTMeta) error {
+					sharedSSTs = append(sharedSSTs, *sst)
+					return nil
+				},
+				false)
 			require.NoError(t, err)
 			require.NoError(t, w.Close())
 

options.go

@@ -20,6 +20,7 @@ import (
 	"github.com/cockroachdb/pebble/internal/keyspan"
 	"github.com/cockroachdb/pebble/internal/manifest"
 	"github.com/cockroachdb/pebble/objstorage/remote"
+	"github.com/cockroachdb/pebble/rangekey"
 	"github.com/cockroachdb/pebble/sstable"
 	"github.com/cockroachdb/pebble/vfs"
 )
@@ -246,9 +247,18 @@ func (o *IterOptions) SpanIterOptions(level manifest.Level) keyspan.SpanIterOpti
 type scanInternalOptions struct {
 	IterOptions
 
+	visitPointKey   func(key *InternalKey, value LazyValue, iterInfo IteratorLevel) error
+	visitRangeDel   func(start, end []byte, seqNum uint64) error
+	visitRangeKey   func(start, end []byte, keys []rangekey.Key) error
+	visitSharedFile func(sst *SharedSSTMeta) error
+
 	// skipSharedLevels skips levels that are shareable (level >=
 	// sharedLevelStart).
 	skipSharedLevels bool
+
+	// includeObsoleteKeys specifies whether keys shadowed by newer internal keys
+	// are exposed. If false, only one internal key per user key is exposed.
+	includeObsoleteKeys bool
 }
 
 // RangeKeyMasking configures automatic hiding of point keys by range keys. A