On 02/07/2014 17:00, Gionatan Danti wrote:
Hi all, it seems that, when using LVM, I/O block transfer size has an hard limit at about 512 KB/iop. Large I/O transfers can be crucial for performance, so I am trying to understand if I can change that. Some info: uname -a (CentOS 6.5 x86_64) Linux blackhole.assyoma.it 2.6.32-431.20.3.el6.x86_64 #1 SMP Thu Jun 19 21:14:45 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux From my understanding, maximum I/O block size for physical device can be tuned from /sys/block/sdX/queue/max_sectors_kb. Some quick iostat -k -x 1 show that, for physical device, the tunable works: # max_sectors_kb=512 (default) dd if=/dev/zero of=/dev/sdd1 bs=2M count=16 oflag=direct iostat -x -k 1 /dev/sdd: avgrq-sz=1024 (1024 sectors = 512KB = max_sectors_kb) # max_sectors_kb=2048 dd if=/dev/zero of=/dev/sdd1 bs=2M count=16 oflag=direct iostat -x -k 1 /dev/sdd: avgrq-sz=2048 (2048 sectors = 1024 KB) dd if=/dev/zero of=/dev/sdd1 bs=4M count=16 oflag=direct iostat -x -k 1 /dev/sdd: avgrq-sz=2730.67 (2730 sectors = ~1350 KB) As you can see, I can't always reach 100% efficienty but I came reasonably close. The problem is that, when using LVM2 (on software raid10), I can not really increase I/O transfer size. Setting both sd*, md* and dm-* to max_sectors_kb=2048 leads to _no_ increase in I/O blocks, while decreasing the max_sectors_kb works properly: # max_sectors_kb=2048 dd if=/dev/zero of=/dev/vg_kvm/TEST bs=2M count=64 oflag=direct iostat -x -k 1 /dev/vg_kvm/TEST: avgrq-sz=1024.00 (it remains as 512KB) # max_sectors_kb=256 dd if=/dev/zero of=/dev/vg_kvm/TEST bs=2M count=64 oflag=direct iostat -x -k 1 /dev/vg_kvm/TEST: avgrq-sz=512.00 (512 sectors = 256KB = max_sectors_kb) Now, two questions: 1) why I see this hard limit? 2) can I change this behavior? Thank you very much.
Anyone with some ideas? :) -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8 _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
