diff --git a/compact.go b/compact.go
index af6ab72df..fe34174a7 100644
--- a/compact.go
+++ b/compact.go
@@ -13,16 +13,24 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
+// Compactor provides compaction against an underlying storage
+// of time series data.
 type Compactor interface {
-	Plan() ([][]string, error)
+	// Plan returns a set of non-overlapping directories that can
+	// be compacted concurrently.
+	// Results returned when compactions are in progress are undefined.
+	Plan(dir string) ([][]string, error)
 
-	Write(dir string, bs ...Block) error
+	// Write persists a Block into a directory.
+	Write(dir string, b Block) error
 
+	// Compact runs compaction against the provided directories. Must
+	// only be called concurrently with results of Plan().
 	Compact(dirs ...string) error
 }
 
+// compactor implements the Compactor interface.
 type compactor struct {
-	dir     string
 	metrics *compactorMetrics
 	opts    *compactorOptions
 }
@@ -63,9 +71,8 @@ type compactorOptions struct {
 	maxBlockRange uint64
 }
 
-func newCompactor(dir string, r prometheus.Registerer, opts *compactorOptions) *compactor {
+func newCompactor(r prometheus.Registerer, opts *compactorOptions) *compactor {
 	return &compactor{
-		dir:     dir,
 		opts:    opts,
 		metrics: newCompactorMetrics(r),
 	}
@@ -79,8 +86,8 @@ type compactionInfo struct {
 
 const compactionBlocksLen = 3
 
-func (c *compactor) Plan() ([][]string, error) {
-	dirs, err := blockDirs(c.dir)
+func (c *compactor) Plan(dir string) ([][]string, error) {
+	dirs, err := blockDirs(dir)
 	if err != nil {
 		return nil, err
 	}
@@ -150,6 +157,7 @@ func mergeBlockMetas(blocks ...Block) (res BlockMeta) {
 
 func (c *compactor) Compact(dirs ...string) (err error) {
 	var blocks []Block
+
 	for _, d := range dirs {
 		b, err := newPersistedBlock(d)
 		if err != nil {
@@ -158,10 +166,16 @@ func (c *compactor) Compact(dirs ...string) (err error) {
 		blocks = append(blocks, b)
 	}
 
-	return c.Write(dirs[0], blocks...)
+	return c.write(dirs[0], blocks...)
 }
 
-func (c *compactor) Write(dir string, blocks ...Block) (err error) {
+func (c *compactor) Write(dir string, b Block) error {
+	return c.write(dir, b)
+}
+
+// write creates a new block that is the union of the provided blocks into dir.
+// It cleans up all files of the old blocks after completing successfully.
+func (c *compactor) write(dir string, blocks ...Block) (err error) {
 	defer func(t time.Time) {
 		if err != nil {
 			c.metrics.failed.Inc()
@@ -169,26 +183,32 @@ func (c *compactor) Write(dir string, blocks ...Block) (err error) {
 		c.metrics.duration.Observe(time.Since(t).Seconds())
 	}(time.Now())
 
-	if err = os.RemoveAll(dir); err != nil {
+	tmp := dir + ".tmp"
+
+	if err = os.RemoveAll(tmp); err != nil {
 		return err
 	}
 
-	if err = os.MkdirAll(dir, 0777); err != nil {
+	if err = os.MkdirAll(tmp, 0777); err != nil {
 		return err
 	}
 
-	chunkw, err := newChunkWriter(chunkDir(dir))
+	chunkw, err := newChunkWriter(chunkDir(tmp))
 	if err != nil {
 		return errors.Wrap(err, "open chunk writer")
 	}
-	indexw, err := newIndexWriter(dir)
+	indexw, err := newIndexWriter(tmp)
 	if err != nil {
 		return errors.Wrap(err, "open index writer")
 	}
 
-	if err = c.write(dir, blocks, indexw, chunkw); err != nil {
+	meta, err := c.populate(blocks, indexw, chunkw)
+	if err != nil {
 		return errors.Wrap(err, "write compaction")
 	}
+	if err = writeMetaFile(tmp, meta); err != nil {
+		return errors.Wrap(err, "write merged meta")
+	}
 
 	if err = chunkw.Close(); err != nil {
 		return errors.Wrap(err, "close chunk writer")
@@ -196,16 +216,28 @@ func (c *compactor) Write(dir string, blocks ...Block) (err error) {
 	if err = indexw.Close(); err != nil {
 		return errors.Wrap(err, "close index writer")
 	}
+
+	if err := renameFile(tmp, dir); err != nil {
+		return errors.Wrap(err, "rename block dir")
+	}
+	for _, b := range blocks[1:] {
+		if err := os.RemoveAll(b.Dir()); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 
-func (c *compactor) write(dir string, blocks []Block, indexw IndexWriter, chunkw ChunkWriter) error {
+// populate fills the index and chunk writers with new data gathered as the union
+// of the provided blocks. It returns meta information for the new block.
+func (c *compactor) populate(blocks []Block, indexw IndexWriter, chunkw ChunkWriter) (*BlockMeta, error) {
 	var set compactionSet
 
 	for i, b := range blocks {
 		all, err := b.Index().Postings("", "")
 		if err != nil {
-			return err
+			return nil, err
 		}
 		// TODO(fabxc): find more transparent way of handling this.
 		if hb, ok := b.(*headBlock); ok {
@@ -219,7 +251,7 @@ func (c *compactor) write(dir string, blocks []Block, indexw IndexWriter, chunkw
 		}
 		set, err = newCompactionMerger(set, s)
 		if err != nil {
-			return err
+			return nil, err
 		}
 	}
 
@@ -234,7 +266,7 @@ func (c *compactor) write(dir string, blocks []Block, indexw IndexWriter, chunkw
 	for set.Next() {
 		lset, chunks := set.At()
 		if err := chunkw.WriteChunks(chunks...); err != nil {
-			return err
+			return nil, err
 		}
 
 		indexw.AddSeries(i, lset, chunks...)
@@ -255,7 +287,7 @@ func (c *compactor) write(dir string, blocks []Block, indexw IndexWriter, chunkw
 		i++
 	}
 	if set.Err() != nil {
-		return set.Err()
+		return nil, set.Err()
 	}
 
 	s := make([]string, 0, 256)
@@ -266,13 +298,13 @@ func (c *compactor) write(dir string, blocks []Block, indexw IndexWriter, chunkw
 			s = append(s, x)
 		}
 		if err := indexw.WriteLabelIndex([]string{n}, s); err != nil {
-			return err
+			return nil, err
 		}
 	}
 
 	for t := range postings.m {
 		if err := indexw.WritePostings(t.name, t.value, postings.get(t)); err != nil {
-			return err
+			return nil, err
 		}
 	}
 	// Write a postings list containing all series.
@@ -281,10 +313,10 @@ func (c *compactor) write(dir string, blocks []Block, indexw IndexWriter, chunkw
 		all[i] = uint32(i)
 	}
 	if err := indexw.WritePostings("", "", newListPostings(all)); err != nil {
-		return err
+		return nil, err
 	}
 
-	return writeMetaFile(dir, &meta)
+	return &meta, nil
 }
 
 type compactionSet interface {
diff --git a/db.go b/db.go
index 463433bc8..78d226833 100644
--- a/db.go
+++ b/db.go
@@ -172,7 +172,7 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 		donec:    make(chan struct{}),
 		stopc:    make(chan struct{}),
 	}
-	db.compactor = newCompactor(dir, r, &compactorOptions{
+	db.compactor = newCompactor(r, &compactorOptions{
 		maxBlockRange: opts.MaxBlockDuration,
 	})
 
@@ -245,7 +245,7 @@ func (db *DB) compact() error {
 
 	// Check for compactions of multiple blocks.
 	for {
-		plans, err := db.compactor.Plan()
+		plans, err := db.compactor.Plan(db.dir)
 		if err != nil {
 			return errors.Wrap(err, "plan compaction")
 		}
@@ -363,7 +363,7 @@ func (db *DB) reloadBlocks() error {
 	for seq, b := range db.seqBlocks {
 		if _, ok := seqBlocks[seq]; !ok {
 			if err := b.Close(); err != nil {
-				return err
+				return errors.Wrapf(err, "closing removed block %d", b.Meta().Sequence)
 			}
 		}
 	}
diff --git a/head.go b/head.go
index 4ddd1957c..969bf3416 100644
--- a/head.go
+++ b/head.go
@@ -149,8 +149,11 @@ func (h *headBlock) Close() error {
 		return err
 	}
 	// Check whether the head block still exists in the underlying dir
-	// or has already been replaced with a compacted version
+	// or has already been replaced with a compacted version or removed.
 	meta, err := readMetaFile(h.dir)
+	if os.IsNotExist(err) {
+		return nil
+	}
 	if err != nil {
 		return err
 	}