do not grow dbsize agressively

Only grow the database size when the high watermark increases.
We also grows the database size a little bit aggressively to
save a few ftruncates.

I have tested this on various environments. The performance impact
is ignorable with 16MB over allocation. Without over allocation,
the performance might decrease 100% when each Tx.Commit needs a new
page on a very slow disk (seek time dominates the total write).

1 files changed, 32 insertions, 0 deletions

diff --git a/db.go b/db.go
index d39c4aa..fd920a0 100644
--- a/db.go
+++ b/db.go
@@ -84,6 +84,7 @@ type DB struct {
 	dataref  []byte // mmap'ed readonly, write throws SEGV
 	data     *[maxMapSize]byte
 	datasz   int
+	filesz   int // current on disk file size
 	meta0    *meta
 	meta1    *meta
 	pageSize int
@@ -655,6 +656,37 @@ func (db *DB) allocate(count int) (*page, error) {
 	return p, nil
 }
 
+// growSize grows the size of the database to the given sz.
+func (db *DB) growSize(sz int) error {
+	if sz <= db.filesz {
+		return nil
+	}
+
+	// over allocate 16MB to avoid calling Truncate aggressively
+	// for efficiency
+	overAllocation := 16 * 1024 * 1024
+	sz = sz + overAllocation
+
+	// do not over allocate
+	if sz > db.datasz {
+		sz = db.datasz
+	}
+
+	// Truncate and fsync to ensure file size metadata is flushed.
+	// https://github.com/boltdb/bolt/issues/284
+	if !db.NoGrowSync && !db.readOnly {
+		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)
+		}
+	}
+
+	db.filesz = sz
+	return nil
+}
+
 func (db *DB) IsReadOnly() bool {
 	return db.readOnly
 }