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    SubjectRe: [PATCH v6 05/11] block: remove split code in blkdev_issue_{discard,write_same}
    On Wed, Oct 21 2015 at 12:02pm -0400,
    Mike Snitzer <snitzer@redhat.com> wrote:

    > On Wed, Oct 14 2015 at 9:27am -0400,
    > Christoph Hellwig <hch@infradead.org> 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).


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