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

[Gionatan Danti <g.danti@assyoma.it>]

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
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