manifest: add blob file rewrite heuristic

Adapt CurrentBlobFileSet to maintain state in support of picking blob files for
rewrite. Specifically, the CurrentBlobFileSet is extended with two heaps. One
heap holds a set of files that are not fully referenced but are considered
'recently created,' ordered within the heap by the blob file creation time. The
other heap holds files that are not fully referenced, ordered within the heap
by the fraction of the file's data that is referenced.

These heaps will be used during compaction picking to pick blob files to
rewrite when value separation space amplification becomes intolerably high.

Informs #112.
Informs #4807.

Author: jbowens

cmd/pebble/db.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"log"
+	"time"
 
 	"github.com/cockroachdb/pebble"
 	"github.com/cockroachdb/pebble/bloom"
@@ -86,6 +87,7 @@ func newPebbleDB(dir string) DB {
 			Enabled:               true,
 			MinimumSize:           512,
 			MaxBlobReferenceDepth: 10,
+			RewriteMinimumAge:     5 * time.Minute,
 		}
 	}
 

compaction_test.go

@@ -64,7 +64,9 @@ func newVersionWithLatest(
 		l0Organizer:     l0Organizer,
 		virtualBackings: manifest.MakeVirtualBackings(),
 	}
-	latest.blobFiles.Init(nil)
+	latest.blobFiles.Init(nil, manifest.BlobRewriteHeuristic{
+		CurrentTime: time.Now,
+	})
 	return v, latest
 }
 

db.go

@@ -2168,7 +2168,7 @@ func (d *DB) Metrics() *Metrics {
 	backingCount, backingTotalSize := d.mu.versions.latest.virtualBackings.Stats()
 	metrics.Table.BackingTableCount = uint64(backingCount)
 	metrics.Table.BackingTableSize = backingTotalSize
-	blobStats := d.mu.versions.latest.blobFiles.Stats()
+	blobStats, _ := d.mu.versions.latest.blobFiles.Stats()
 	d.mu.versions.logUnlock()
 	metrics.BlobFiles.LiveCount = blobStats.Count
 	metrics.BlobFiles.LiveSize = blobStats.PhysicalSize

external_test.go

@@ -199,6 +199,7 @@ func buildSeparatedValuesDB(
 			Enabled:               true,
 			MinimumSize:           50,
 			MaxBlobReferenceDepth: 10,
+			RewriteMinimumAge:     15 * time.Minute,
 		}
 	}
 	o.Levels[0].BlockSize = 32 << 10       // 32 KB