Add full Snapshot support

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

block.go

@@ -1,4 +1,5 @@
 // Copyright 2017 The Prometheus Authors
+
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
@@ -52,13 +53,13 @@ type DiskBlock interface {
 type Block interface {
 	DiskBlock
 	Queryable
+	Snapshottable
 }
 
 // headBlock is a regular block that can still be appended to.
 type headBlock interface {
 	Block
 	Appendable
-	Snapshottable
 }
 
 // Snapshottable defines an entity that can be backedup online.
@@ -278,6 +279,42 @@ Outer:
 	return writeMetaFile(pb.dir, &pb.meta)
 }
 
+func (pb *persistedBlock) Snapshot(dir string) error {
+	blockDir := filepath.Join(dir, pb.meta.ULID.String())
+	if err := os.MkdirAll(blockDir, 0777); err != nil {
+		return errors.Wrap(err, "create snapshot block dir")
+	}
+
+	chunksDir := chunkDir(blockDir)
+	if err := os.MkdirAll(chunksDir, 0777); err != nil {
+		return errors.Wrap(err, "create snapshot chunk dir")
+	}
+
+	// Hardlink meta, index and tombstones
+	filenames := []string{metaFilename, indexFilename, tombstoneFilename}
+	for _, fname := range filenames {
+		if err := os.Link(filepath.Join(pb.dir, fname), filepath.Join(blockDir, fname)); err != nil {
+			return errors.Wrapf(err, "create snapshot %s", fname)
+		}
+	}
+
+	// Hardlink the chunks
+	curChunkDir := chunkDir(pb.dir)
+	files, err := ioutil.ReadDir(curChunkDir)
+	if err != nil {
+		return errors.Wrap(err, "ReadDir the current chunk dir")
+	}
+
+	for _, f := range files {
+		err := os.Link(filepath.Join(curChunkDir, f.Name()), filepath.Join(chunksDir, f.Name()))
+		if err != nil {
+			return errors.Wrap(err, "hardlink a chunk")
+		}
+	}
+
+	return nil
+}
+
 func chunkDir(dir string) string { return filepath.Join(dir, "chunks") }
 func walDir(dir string) string   { return filepath.Join(dir, "wal") }
 

db.go

@@ -121,7 +121,8 @@ type DB struct {
 	stopc    chan struct{}
 
 	// cmtx is used to control compactions and deletions.
-	cmtx sync.Mutex
+	cmtx       sync.Mutex
+	compacting bool
 }
 
 type dbMetrics struct {
@@ -200,12 +201,13 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 	}
 
 	db = &DB{
-		dir:      dir,
-		logger:   l,
-		opts:     opts,
-		compactc: make(chan struct{}, 1),
-		donec:    make(chan struct{}),
-		stopc:    make(chan struct{}),
+		dir:        dir,
+		logger:     l,
+		opts:       opts,
+		compactc:   make(chan struct{}, 1),
+		donec:      make(chan struct{}),
+		stopc:      make(chan struct{}),
+		compacting: true,
 	}
 	db.metrics = newDBMetrics(db, r)
 
@@ -528,27 +530,31 @@ func (db *DB) Close() error {
 	return merr.Err()
 }
 
-// DisableCompactions disables compactions.
-func (db *DB) DisableCompactions() error {
-	db.stopc <- struct{}{} // TODO: Can this block?
-	db.cmtx.Lock()
-	return nil
-}
+// ToggleCompactions toggles compactions and returns if compactions are on or not.
+func (db *DB) ToggleCompactions() bool {
+	if db.compacting {
+		db.cmtx.Lock()
+		db.compacting = false
+		return false
+	}
 
-// EnableCompactions enables compactions.
-func (db *DB) EnableCompactions() error {
 	db.cmtx.Unlock()
-	return nil
+	db.compacting = true
+	return true
 }
 
 // Snapshot writes the current headBlock snapshots to snapshots directory.
 func (db *DB) Snapshot(dir string) error {
-	db.headmtx.RLock()
-	heads := db.heads[:]
-	db.headmtx.RUnlock()
+	db.mtx.Lock() // To block any appenders.
+	defer db.mtx.Unlock()
 
-	for _, h := range heads {
-		if err := h.Snapshot(dir); err != nil {
+	db.cmtx.Lock()
+	defer db.cmtx.Unlock()
+
+	blocks := db.blocks[:]
+	for _, b := range blocks {
+		db.logger.Log("msg", "compacting block", "block", b.Dir())
+		if err := b.Snapshot(dir); err != nil {
 			return errors.Wrap(err, "error snapshotting headblock")
 		}
 	}

head.go

@@ -263,11 +263,10 @@ Outer:
 }
 
 // Snapshot persists the current state of the headblock to the given directory.
+// TODO(gouthamve): Snapshot must be called when there are no active appenders.
+// This has been ensured by acquiring a Lock on DB.mtx, but this limitation should
+// be removed in the future.
 func (h *HeadBlock) Snapshot(snapshotDir string) error {
-	// Needed to stop any appenders.
-	h.mtx.Lock()
-	defer h.mtx.Unlock()
-
 	if h.meta.Stats.NumSeries == 0 {
 		return nil
 	}

index.go

@@ -39,6 +39,8 @@ const (
 	indexFormatV1 = 1
 )
 
+const indexFilename = "index"
+
 const compactionPageBytes = minSectorSize * 64
 
 type indexWriterSeries struct {
@@ -138,7 +140,7 @@ func newIndexWriter(dir string) (*indexWriter, error) {
 	if err != nil {
 		return nil, err
 	}
-	f, err := os.OpenFile(filepath.Join(dir, "index"), os.O_CREATE|os.O_WRONLY, 0666)
+	f, err := os.OpenFile(filepath.Join(dir, indexFilename), os.O_CREATE|os.O_WRONLY, 0666)
 	if err != nil {
 		return nil, err
 	}