use fsync to force persisting the meta file. (#573)

* use fsync to force persisting the meta file.

Signed-off-by: Krasi Georgiev <kgeorgie@redhat.com>

CHANGELOG.md

@@ -2,6 +2,7 @@
  - [REMOVED] `chunks.NewReader` is removed as it wasn't used anywhere.
  - [REMOVED] `FromData` is considered unused so was removed.
  - [FEATURE] Added option WALSegmentSize -1 to disable the WAL.
+ - [BUGFIX] Fsync the meta file to persist it on disk to avoid data loss in case of a host crash.
 
 ## 0.6.1
   - [BUGFIX] Update `last` after appending a non-overlapping chunk in `chunks.MergeOverlappingChunks`. [#539](https://github.com/prometheus/tsdb/pull/539)

block.go

@@ -251,6 +251,11 @@ func writeMetaFile(dir string, meta *BlockMeta) error {
 		merr.Add(f.Close())
 		return merr.Err()
 	}
+	// Force the kernel to persist the file on disk to avoid data loss if the host crashes.
+	if merr.Add(f.Sync()); merr.Err() != nil {
+		merr.Add(f.Close())
+		return merr.Err()
+	}
 	if err := f.Close(); err != nil {
 		return err
 	}