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authorMikulas Patocka <mpatocka@redhat.com>2010-08-12 07:14:02 +0400
committerAlasdair G Kergon <agk@redhat.com>2010-08-12 07:14:02 +0400
commitb1d5552838334c600b068c9c8cc18638e5a8cb47 (patch)
tree789e435ab60e6956abffc15b4da85fe1e5b1714c /drivers/md/dm-ioctl.c
parent4a0b4ddf261fc89c050fe0a10ec57a61251d7ac0 (diff)
downloadlinux-b1d5552838334c600b068c9c8cc18638e5a8cb47.tar.xz
dm snapshot: implement merge
Implement merge method for the snapshot origin to improve read performance. Without merge method, dm asks the upper layers to submit smallest possible bios --- one page. Submitting such small bios impacts performance negatively when reading or writing the origin device. Without this patch, CPU consumption when reading the origin on lvm on md-raid0 was 6 to 12%, with this patch, it drops to 1 to 4%. Note: in my testing, it actually degraded performance in some settings, I traced it to Maxtor disks having problems with > 512-sector requests. Reducing the number of sectors to /sys/block/sd*/queue/max_sectors_kb to 256 fixed the read performance. I think we don't have to care about weird disks that actually degrade performance because of large requests being sent to them. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
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