Address comments

Signed-off-by: Fabian Reinartz <freinartz@google.com>

docs/format/wal.md

@@ -1,6 +1,6 @@
 # WAL Disk Format
 
-The write ahead log operates in segments that that are numbered and sequential,
+The write ahead log operates in segments that are numbered and sequential,
 e.g. `000000`, `000001`, `000002`, etc., and are limited to 128MB by default.
 A segment is written to in pages of 32KB. Only the last page of the most recent segment
 may be partial. A WAL record is an opaque byte slice that gets split up into sub-records
@@ -17,13 +17,21 @@ Notable deviations are that the record fragment is encoded as:
 └───────────┴──────────┴────────────┴──────────────┘
 ```
 
+The type flag has the following states:
+
+* `0`: rest of page will be empty
+* `1`: a full record encoded in a single fragment
+* `2`: first fragment of a record
+* `3`: middle fragment of a record
+* `4`: final fragment of a record
+
 ## Record encoding
 
 The records written to the write ahead log are encoded as follows:
 
 ### Series records
 
-Series records encode the labels that identifier a series and its unique ID.
+Series records encode the labels that identifies a series and its unique ID.
 
 ```
 ┌────────────────────────────────────────────┐

fileutil/fileutil.go

@@ -27,9 +27,6 @@ func ReadDir(dirpath string) ([]string, error) {
 
 // Rename safely renames a file.
 func Rename(from, to string) error {
-	if err := os.RemoveAll(to); err != nil {
-		return err
-	}
 	if err := os.Rename(from, to); err != nil {
 		return err
 	}

wal/wal.go

@@ -140,7 +140,7 @@ func OpenReadSegment(fn string) (*Segment, error) {
 
 // WAL is a write ahead log that stores records in segment files.
 // It must be read from start to end once before logging new data.
-// If an erroe occurs during read, the repair procedure must be called
+// If an error occurs during read, the repair procedure must be called
 // before it's safe to do further writes.
 //
 // Segments are written to in pages of 32KB, with records possibly split