I guess the list dropped this mail. Sending again.
-aneesh
--- Begin Message ---
Aneesh Kumar K.V wrote:
Hi All,
I looked at the delalloc and reservation differences that Valerie was
observing.
Below is my understanding. I am not sure whether the below will result
in higher fragmentation that Eric Sandeen is observing. I guess it
should not. Even
though the reservation gets discarded during the clear inode due to
memory pressure
the request for new reservation should get the blocks nearby and not
break extents right ?
any how below is the simple case.
without delalloc the blocks are requested during prepare_write/write_begin.
That means we enter ext4_new_blocks_old which will call
ext4_try_to_allocate_with_rsv.
Now if there is no reservation for this inode a new one will be
allocated. After
using the blocks this reservation is destroyed during the close via
ext4_release_file
With delalloc the blocks are not requested until we hit
writeback/ext4_da_writepages
That means if we create new file and close them the reservation will be
discarded
during close via ext4_release_file.( Actually there will be nothing to
clear)
Now when we do a sync/or write back. We try to get the block, the inode
will
request for new reservation. This reservation is not discarded untill we
call clear_inode
and that results in the behavior we are seeing.
Free blocks: 1440-8191, 8194-8199, 8202-8207, 8210-8215, 8218-8223,
8226-8231, 8234-8239, 8242-8247, 8250-8255, 8258-8263, 8266-8271,
8274-8279, 8282-8287, 8290-8295, 8298-8303, 8306-8311, 8314-8319,
8322-8327, 8330-8335, 8338-8343, 8346-12799
So now the question is where do we discard the reservation in case of
delalloc.
-
with respect to mballoc we are not seeing this because we are doing
allocation from group prealloc list which is per cpu.
For most the case we have EXT4_MB_HINT_GROUP_ALLOC set in mballoc.
In ext4_mb_group_or_file i already have a FIXME!! regarding this.
currently we have
/* request is so large that we don't care about
* streaming - it overweights any possible seek */
if (ac->ac_o_ex.fe_len >= sbi->s_mb_large_req)
return;
/* FIXME!!
* is this >= considering the above ?
*/
if (ac->ac_o_ex.fe_len >= sbi->s_mb_small_req)
return;
.....
......
/* we're going to use group allocation */
ac->ac_flags |= EXT4_MB_HINT_GROUP_ALLOC;
........
.........
So for small size we have the EXT4_MB_HINT_GROUP_ALLOC set . Now if
i change the the line below FIXME!! to <= , that will force
small size to use inode prealloc and that cause
Free blocks: 1442-1443, 1446-1447, 1450-1451, 1454-1455, 1458-1459, 1462-1463, 1466-1467, 1470-1471, 1474-1475, 1478-1479, 1482-1483, 1486-1487, 1490-1491, 1494-1495, 1498-1499, 1502-1503, 1506-1507, 1510-1511, 1514-1515, 1518-12799
So the problem is generic.
-aneesh
--- End Message ---
