Add DB.NoGrowSync flag.

This commit adds the DB.NoGrowSync flag to optionally revert mmap()
calls to how they were implemented before the ext3/ext4 fix. When
NoGrowSync is true, remapping the data file will not force the file
system to resize it immediately. This works for non-ext3/4 file
systems.

The default value of NoGrowSync is false so it is still safe for
ext3/ext4 file systems by default.

See also: boltdb#284

bolt_unix.go

@@ -44,11 +44,13 @@ func funlock(f *os.File) error {
 func mmap(db *DB, sz int) error {
  // Truncate and fsync to ensure file size metadata is flushed.
  // https:/boltdb/bolt/issues/284
- if err := db.file.Truncate(int64(sz)); err != nil {
- return fmt.Errorf("file resize error: %s", err)
- }
- if err := db.file.Sync(); err != nil {
- return fmt.Errorf("file sync error: %s", err)
+ if !db.NoGrowSync {
+ if err := db.file.Truncate(int64(sz)); err != nil {
+ return fmt.Errorf("file resize error: %s", err)
+ }
+ if err := db.file.Sync(); err != nil {
+ return fmt.Errorf("file sync error: %s", err)
+ }
  }
 
  // Map the data file to memory.

db.go

@@ -55,6 +55,14 @@ type DB struct {
  // THIS IS UNSAFE. PLEASE USE WITH CAUTION.
  NoSync bool
 
+ // When true, skips the truncate call when growing the database.
+ // Setting this to true is only safe on non-ext3/ext4 systems.
+ // Skipping truncation avoids preallocation of hard drive space and
+ // bypasses a truncate() and fsync() syscall on remapping.
+ //
+ // https:/boltdb/bolt/issues/284
+ NoGrowSync bool
+
  // MaxBatchSize is the maximum size of a batch. Default value is
  // copied from DefaultMaxBatchSize in Open.
  //
@@ -123,6 +131,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
  if options == nil {
  options = DefaultOptions
  }
+ db.NoGrowSync = options.NoGrowSync
 
  // Set default values for later DB operations.
  db.MaxBatchSize = DefaultMaxBatchSize
@@ -613,12 +622,16 @@ type Options struct {
  // When set to zero it will wait indefinitely. This option is only
  // available on Darwin and Linux.
  Timeout time.Duration
+
+ // Sets the DB.NoGrowSync flag before memory mapping the file.
+ NoGrowSync bool
 }
 
 // DefaultOptions represent the options used if nil options are passed into Open().
 // No timeout is used which will cause Bolt to wait indefinitely for a lock.
 var DefaultOptions = &Options{
- Timeout: 0,
+ Timeout: 0,
+ NoGrowSync: false,
 }
 
 // Stats represents statistics about the database.