Re: Ext4 and xfs problems in dm-thin on allocation and discard

[Dave Chinner <david@fromorbit.com>]

On Wed, Jun 20, 2012 at 02:11:31PM +0200, Spelic wrote:
> Ok guys, I think I found the bug. One or more bugs.
> 
> 
> Pool has chunksize 1MB.
> In sysfs the thin volume has: queue/discard_max_bytes and
> queue/discard_granularity are 1048576 .
> And it has discard_alignment = 0, which based on sysfs-block
> documentation is correct (a less misleading name would have been
> discard_offset imho).
> Here is the blktrace from ext4 fstrim:
> ...
> 252,9   17      498     0.030466556   841  Q   D 19898368 + 2048 [fstrim]
> 252,9   17      499     0.030467501   841  Q   D 19900416 + 2048 [fstrim]
> 252,9   17      500     0.030468359   841  Q   D 19902464 + 2048 [fstrim]
> 252,9   17      501     0.030469313   841  Q   D 19904512 + 2048 [fstrim]
> 252,9   17      502     0.030470144   841  Q   D 19906560 + 2048 [fstrim]
> 252,9   17      503     0.030471381   841  Q   D 19908608 + 2048 [fstrim]
> 252,9   17      504     0.030472473   841  Q   D 19910656 + 2048 [fstrim]
> 252,9   17      505     0.030473504   841  Q   D 19912704 + 2048 [fstrim]
> 252,9   17      506     0.030474561   841  Q   D 19914752 + 2048 [fstrim]
> 252,9   17      507     0.030475571   841  Q   D 19916800 + 2048 [fstrim]
> 252,9   17      508     0.030476423   841  Q   D 19918848 + 2048 [fstrim]
> 252,9   17      509     0.030477341   841  Q   D 19920896 + 2048 [fstrim]
> 252,9   17      510     0.034299630   841  Q   D 19922944 + 2048 [fstrim]
> 252,9   17      511     0.034306880   841  Q   D 19924992 + 2048 [fstrim]
> 252,9   17      512     0.034307955   841  Q   D 19927040 + 2048 [fstrim]
> 252,9   17      513     0.034308928   841  Q   D 19929088 + 2048 [fstrim]
> 252,9   17      514     0.034309945   841  Q   D 19931136 + 2048 [fstrim]
> 252,9   17      515     0.034311007   841  Q   D 19933184 + 2048 [fstrim]
> 252,9   17      516     0.034312008   841  Q   D 19935232 + 2048 [fstrim]
> 252,9   17      517     0.034313122   841  Q   D 19937280 + 2048 [fstrim]
> 252,9   17      518     0.034314013   841  Q   D 19939328 + 2048 [fstrim]
> 252,9   17      519     0.034314940   841  Q   D 19941376 + 2048 [fstrim]
> 252,9   17      520     0.034315835   841  Q   D 19943424 + 2048 [fstrim]
> 252,9   17      521     0.034316662   841  Q   D 19945472 + 2048 [fstrim]
> 252,9   17      522     0.034317547   841  Q   D 19947520 + 2048 [fstrim]
> ...
> 
> Here is the blktrace from xfs fstrim:
> 252,12  16        1     0.000000000   554  Q   D 96 + 2048 [fstrim]
> 252,12  16        2     0.000010149   554  Q   D 2144 + 2048 [fstrim]
> 252,12  16        3     0.000011349   554  Q   D 4192 + 2048 [fstrim]
> 252,12  16        4     0.000012584   554  Q   D 6240 + 2048 [fstrim]
> 252,12  16        5     0.000013685   554  Q   D 8288 + 2048 [fstrim]
> 252,12  16        6     0.000014660   554  Q   D 10336 + 2048 [fstrim]
> 252,12  16        7     0.000015707   554  Q   D 12384 + 2048 [fstrim]
> 252,12  16        8     0.000016692   554  Q   D 14432 + 2048 [fstrim]
> 252,12  16        9     0.000017594   554  Q   D 16480 + 2048 [fstrim]
> 252,12  16       10     0.000018539   554  Q   D 18528 + 2048 [fstrim]
> 252,12  16       11     0.000019434   554  Q   D 20576 + 2048 [fstrim]
> 252,12  16       12     0.000020879   554  Q   D 22624 + 2048 [fstrim]
> 252,12  16       13     0.000021856   554  Q   D 24672 + 2048 [fstrim]
> 252,12  16       14     0.000022786   554  Q   D 26720 + 2048 [fstrim]
> 252,12  16       15     0.000023699   554  Q   D 28768 + 2048 [fstrim]
> 252,12  16       16     0.000024672   554  Q   D 30816 + 2048 [fstrim]
> 252,12  16       17     0.000025467   554  Q   D 32864 + 2048 [fstrim]
> 252,12  16       18     0.000026374   554  Q   D 34912 + 2048 [fstrim]
> 252,12  16       19     0.000027194   554  Q   D 36960 + 2048 [fstrim]
> 252,12  16       20     0.000028137   554  Q   D 39008 + 2048 [fstrim]
> 252,12  16       21     0.000029524   554  Q   D 41056 + 2048 [fstrim]
> 252,12  16       22     0.000030479   554  Q   D 43104 + 2048 [fstrim]
> 252,12  16       23     0.000031306   554  Q   D 45152 + 2048 [fstrim]
> 252,12  16       24     0.000032134   554  Q   D 47200 + 2048 [fstrim]
> 252,12  16       25     0.000032964   554  Q   D 49248 + 2048 [fstrim]
> 252,12  16       26     0.000033794   554  Q   D 51296 + 2048 [fstrim]
> 
> 
> As you can see, while ext4 correctly aligns the discards to 1MB, xfs
> does not.

XFs just sends a large extent to blkdev_issue_discard(), and cares
nothing about discard alignment or granularity.

> It looks like an fstrim or xfs bug: they don't look at
> discard_alignment (=0 ... a less misleading name would be
> discard_offset imho) + discard_granularity (=1MB) and they don't
> base alignments on those.

It looks like blkdev_issue_discard() has reduced each discard to
bios of a single "granule" (1MB), and not aligned them, hence they
are ignore by dm-thinp.

what are the discard parameters exposed by dm-thinp in
/sys/block/<thinp-blkdev>/queue/discard*

It looks to me that dmthinp might be setting discard_max_bytes to
1MB rather than discard_granularity. Looking at dm-thin.c:

static void set_discard_limits(struct pool *pool, struct queue_limits *limits)
{
        /*
         * FIXME: these limits may be incompatible with the pool's data device
         */
        limits->max_discard_sectors = pool->sectors_per_block;

        /*
         * This is just a hint, and not enforced.  We have to cope with
         * bios that overlap 2 blocks.
         */
        limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
        limits->discard_zeroes_data = pool->pf.zero_new_blocks;
}


Yes - discard_max_bytes == discard_granularity, and so
blkdev_issue_discard fails to align the request properly. As it is,
setting discard_max_bytes to the thinp block size is silly - it
means you'll never get range requests, and we sent a discard for
every single block in a range rather than having the thinp code
iterate over a range itself.

i.e. this is not a filesystem bug that is causing the problem....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com