Re: [PATCH v6 05/11] block: remove split code in blkdev_issue_{discard,write_same}
From: Mike Snitzer
Date: Wed Oct 21 2015 - 14:21:08 EST
On Wed, Oct 21 2015 at 1:33pm -0400,
Ming Lin <mlin@xxxxxxxxxx> wrote:
> On Wed, 2015-10-21 at 12:19 -0400, Mike Snitzer wrote:
> > On Wed, Oct 21 2015 at 12:02pm -0400,
> > Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> >
> > > On Wed, Oct 14 2015 at 9:27am -0400,
> > > Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > >
> > > > On Tue, Oct 13, 2015 at 10:44:11AM -0700, Ming Lin wrote:
> > > > > I just did a quick test with a Samsung 900G NVMe device.
> > > > > mkfs.xfs is OK on 4.3-rc5.
> > > > >
> > > > > What's your device model? I may find a similar one to try.
> > > >
> > > > This is a HGST Ultrastar SN100
> > > >
> > > > Analsys and tentativ fix below:
> > > >
> > > > blktrace for before the commit:
> > > >
> > > > 259,0 1 2 0.000002543 2394 G D 0 + 8388607 [mkfs.xfs]
> > > > 259,0 1 3 0.000008230 2394 I D 0 + 8388607 [mkfs.xfs]
> > > > 259,0 1 4 0.000031090 207 D D 0 + 8388607 [kworker/1:1H]
> > > > 259,0 1 5 0.000044869 2394 Q D 8388607 + 8388607 [mkfs.xfs]
> > > > 259,0 1 6 0.000045992 2394 G D 8388607 + 8388607 [mkfs.xfs]
> > > > 259,0 1 7 0.000049559 2394 I D 8388607 + 8388607 [mkfs.xfs]
> > > > 259,0 1 8 0.000061551 207 D D 8388607 + 8388607 [kworker/1:1H]
> > > >
> > > > .. and so on.
> > > >
> > > > blktrace with the commit:
> > > >
> > > > 259,0 2 1 0.000000000 1228 Q D 0 + 4194304 [mkfs.xfs]
> > > > 259,0 2 2 0.000002543 1228 G D 0 + 4194304 [mkfs.xfs]
> > > > 259,0 2 3 0.000010080 1228 I D 0 + 4194304 [mkfs.xfs]
> > > > 259,0 2 4 0.000082187 267 D D 0 + 4194304 [kworker/2:1H]
> > > > 259,0 2 5 0.000224869 1228 Q D 4194304 + 4194304 [mkfs.xfs]
> > > > 259,0 2 6 0.000225835 1228 G D 4194304 + 4194304 [mkfs.xfs]
> > > > 259,0 2 7 0.000229457 1228 I D 4194304 + 4194304 [mkfs.xfs]
> > > > 259,0 2 8 0.000238507 267 D D 4194304 + 4194304 [kworker/2:1H]
> > > >
> > > > So discards are smaller, but better aligned. Now if I tweak a single
> > > > line in blk-lib.c to be able to use all of bi_size I get the old I/O
> > > > pattern back and everything works fine again:
> > > >
> > > > diff --git a/block/blk-lib.c b/block/blk-lib.c
> > > > index bd40292..65b61dc 100644
> > > > --- a/block/blk-lib.c
> > > > +++ b/block/blk-lib.c
> > > > @@ -82,7 +82,7 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> > > > break;
> > > > }
> > > >
> > > > - req_sects = min_t(sector_t, nr_sects, MAX_BIO_SECTORS);
> > > > + req_sects = min_t(sector_t, nr_sects, UINT_MAX >> 9);
> > > > end_sect = sector + req_sects;
> > > >
> > > > bio->bi_iter.bi_sector = sector;
> > >
> > > Can we change UINT_MAX >> 9 to rounddown to the first factor of
> > > minimum_io_size?
> > >
> > > That should work for all devices and for dm-thinp (and dm-cache) in
> > > particular will ensure that all discards that are issued will be a
> > > multiple of the underlying device's blocksize.
> >
> > Jeff Moyer pointed out having req_sects be a factor of
> > discard_granularity makes more sense. And I agree. Same difference in
> > the end (since dm-thinp sets discard_granularity to the thinp
> > blocksize).
>
> An old version of this patch did use discard_granularity
> https://www.redhat.com/archives/dm-devel/2015-August/msg00000.html
>
> But you didn't agree.
> https://www.redhat.com/archives/dm-devel/2015-August/msg00001.html
>
> Maybe we can re-add discard_granularity now?
I disagreed on a more generic level than discard_granularity shaping the
split boundary.
But we are where we are. If we're going to split (due to 32-bit limits
in bio->bi_iter.bi_size) then we should at least do so in terms of the
support discard_granularity.
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