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, 8 insertions, 2 deletions

diff --git a/db_test.go b/db_test.go
index ae21938..0f89c33 100644
--- a/db_test.go
+++ b/db_test.go
@@ -100,6 +100,8 @@ func TestOpen_Size(t *testing.T) {
 	path := db.Path()
 	defer db.Close()
 
+	pagesize := db.Info().PageSize
+
 	// Insert until we get above the minimum 4MB size.
 	ok(t, db.Update(func(tx *bolt.Tx) error {
 		b, _ := tx.CreateBucketIfNotExists([]byte("data"))
@@ -127,7 +129,8 @@ func TestOpen_Size(t *testing.T) {
 	}
 
 	// Compare the original size with the new size.
-	if sz != newSz {
+	// db size might increase by a few page sizes due to the new small update.
+	if sz < newSz-5*int64(pagesize) {
 		t.Fatalf("unexpected file growth: %d => %d", sz, newSz)
 	}
 }
@@ -144,6 +147,8 @@ func TestOpen_Size_Large(t *testing.T) {
 	path := db.Path()
 	defer db.Close()
 
+	pagesize := db.Info().PageSize
+
 	// Insert until we get above the minimum 4MB size.
 	var index uint64
 	for i := 0; i < 10000; i++ {
@@ -177,7 +182,8 @@ func TestOpen_Size_Large(t *testing.T) {
 	}
 
 	// Compare the original size with the new size.
-	if sz != newSz {
+	// db size might increase by a few page sizes due to the new small update.
+	if sz < newSz-5*int64(pagesize) {
 		t.Fatalf("unexpected file growth: %d => %d", sz, newSz)
 	}
 }