I'm bisecting now, there are a couple of patches from Ming in
the area of the bio splitting code...
CC'ing Ming, Linux-block and Linux-nvme as iser is identical to nvme
wrt the virtual boundary so I think nvme will break as well.
Bisection reveals that this one is the culprit:
commit 52cc6eead9095e2faf2ec7afc013aa3af1f01ac5
Author: Ming Lei <ming.lei@xxxxxxxxxxxxx>
Date: Thu Sep 17 09:58:38 2015 -0600
block: blk-merge: fast-clone bio when splitting rw bios
biovecs has become immutable since v3.13, so it isn't necessary
to allocate biovecs for the new cloned bios, then we can save
one extra biovecs allocation/copy, and the allocation is often
not fixed-length and a bit more expensive.
For example, if the 'max_sectors_kb' of null blk's queue is set
as 16(32 sectors) via sysfs just for making more splits, this patch
can increase throught about ~70% in the sequential read test over
null_blk(direct io, bs: 1M).
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: Ming Lin <ming.l@xxxxxxxxxxxxxxx>
Cc: Dongsu Park <dpark@xxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
This fixes a performance regression introduced by commit 54efd50bfd,
and allows us to take full advantage of the fact that we have immutable
bio_vecs. Hand applied, as it rejected violently with commit
5014c311baa2.
Signed-off-by: Jens Axboe <axboe@xxxxxx>
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