From: Mikulas Patocka Date: Thu, 12 Aug 2010 03:14:02 +0000 (+0100) Subject: dm snapshot: implement merge X-Git-Tag: v2.6.36-rc1~51^2~17 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b1d5552838334c600b068c9c8cc18638e5a8cb47;p=pandora-kernel.git 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 Signed-off-by: Alasdair G Kergon --- Reading git-diff-tree failed