On 2011-02-09, at 23:33, Tao Ma [off-list ref] wrote:

After I delete some big image files and re-run the trim,
it is still much faster than iterating the whole disk.
/dev/sdb2             108G   25G   78G  24% /mnt/ext4

[root@boyu-tm test]# time ./ftrim /mnt/ext4/a
real    0m0.513s
user    0m0.000s
sys    0m0.069s

Excellent results. 

+#define EXT4_GROUP_INFO_NEED_INIT_BIT        0
+#define EXT4_GROUP_INFO_WAS_TRIMMED_BIT        1

#define EXT4_MB_GRP_NEED_INIT(grp)    \
   (test_bit(EXT4_GROUP_INFO_NEED_INIT_BIT, &((grp)->bb_state)))
+#define EXT4_MB_GRP_HAS_BEEN_TRIMMED(grp)    \

For consistency, it would be better to call this:

EXT4_MB_GRP_WAS_TRIMMED(grp)

And also add and use:

EXT4_MB_GRP_SET_TRIMMED(grp)

And 

EXT4_MB_GRP_CLEAR_TRIMMED(grp)

In the code. Then you can add Reviewed-by: on this patch. 

Cheers, Andreas