On Wed, Jun 20 2012 at 6:53pm -0400,
Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> 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.
So 2 different issues:
1) blkdev_issue_discard isn't properly aligning
2) thinp should accept larger discards (up to the stacked
discard_max_bytes rather than setting an override)
> i.e. this is not a filesystem bug that is causing the problem....
Paolo Bonzini fixed blkdev_issue_discard to properly align some time
ago; unfortunately the patches slipped through the cracks (cc'ing Paolo,
Jens, and Christoph).
Here are references to Paolo's patches:
0/2 https://lkml.org/lkml/2012/3/14/323
1/2 https://lkml.org/lkml/2012/3/14/324
2/2 https://lkml.org/lkml/2012/3/14/325
Patch 2/2 specifically addresses the case where:
discard_max_bytes == discard_granularity
Paolo, any chance you could resend to Jens (maybe with hch's comments on
patch#2 accounted for)? Also, please add hch's Reviewed-by when
reposting.
(would love to see this fixed for 3.5-rcX but if not 3.6 it is?)
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