Improve error messages, create regular dir for block

block.go

@@ -114,18 +114,20 @@ type mmapFile struct {
 }
 
 func openMmapFile(path string) (*mmapFile, error) {
-	f, err := fileutil.TryLockFile(path, os.O_RDONLY, 0666)
+	// We have to open the file in RDWR for the lock to work with fileutil.
+	// TODO(fabxc): use own flock call that supports multi-reader.
+	f, err := fileutil.TryLockFile(path, os.O_RDWR, 0666)
 	if err != nil {
-		return nil, err
+		return nil, errors.Wrap(err, "try lock file")
 	}
 	info, err := f.Stat()
 	if err != nil {
-		return nil, err
+		return nil, errors.Wrap(err, "stat")
 	}
 
 	b, err := mmap(f.File, int(info.Size()))
 	if err != nil {
-		return nil, err
+		return nil, errors.Wrap(err, "mmap")
 	}
 
 	return &mmapFile{f: f, b: b}, nil

compact.go

@@ -134,7 +134,7 @@ func (c *compactor) compact(dir string, blocks ...Block) (err error) {
 			return err
 		}
 	}
-	if err = fileutil.CreateDirAll(dir); err != nil {
+	if err = os.MkdirAll(dir, 0755); err != nil {
 		return err
 	}
 

db.go

@@ -113,8 +113,8 @@ func Open(dir string, logger log.Logger, opts *Options) (db *DB, err error) {
 			return nil, err
 		}
 	}
-	var r prometheus.Registerer
+	// var r prometheus.Registerer
-	// r := prometheus.DefaultRegisterer
+	r := prometheus.DefaultRegisterer
 
 	if opts == nil {
 		opts = DefaultOptions
@@ -393,6 +393,8 @@ func (a *dbAppender) Add(ref uint64, t int64, v float64) error {
 	if gen != a.gen {
 		return ErrNotFound
 	}
+	a.db.metrics.samplesAppended.Inc()
+
 	return a.head.Add(ref, t, v)
 }