[linux-lvm] LVM and I/O block size (max_sectors_kb)

[Gionatan Danti]

> 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.

Hi all,
sorry for bumping up this thread.

Anyone has an explanation for what I see? I am missing something?

Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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