Re: [Xen-users] blk[front|back] does not hand over minimum and optimal_io_size to domU

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Wed, Feb 23, 2011 at 02:26:41PM +0100, Adi Kriegisch wrote:
> Dear all,
> 
> I investigated some serious performance drop between Dom0 and DomU with
> LVM on top of RAID6 and blkback devices.
> While I have around 130MB/s write performance in Dom0, I only get 30MB/s in
> DomU. Inspecting this with dstat/iostat revealed that I have a read rate of
> about 17-25MB/s while writing with aroung 40MB/s.
> The reading only occurs on the disk devices assembled to the RAID6 not the
> md device itself. So this is related to RAID6 activity only.
> The reason for this is recalculation of checksums due to a too small
> optimal_io_size:
> On Dom0:
> blockdev --getiomin /dev/space/test
> 524288 (which is chunk size)
> blockdev --getioopt /dev/space/test
> 3145728 (which is 6*chunk size)
> 
> On DomU:
> blockdev --getiomin /dev/xvdb1
> 512
> blockdev --getioopt /dev/xvdb1
> 0 (so the kernel will use 1MB by default, IIRC)
> 
> minimum_io_size -- if not set -- is hardware block size which seems to be
> set to 512 in xlvbd_init_blk_queue (blkfront.c). Btw: blockdev --getbsz
> /dev/space/test gives 4096 on Dom0 while DomU reports 512.
> 
> I can somehow mitigate the issue by using a way smaller chunk size but this
> is IMHO just working around the issue.
> 
> Is this a bug or a regression? Or does this happen to anyone using RAID6
> (and probably RAID5 as well) and noone noticed the drop until now?
> Is there any way to work around this issue?
> 
> Thanks,
>       Adi Kriegisch
> 
> PS: I am using a stock Debian/Squeeze kernel on top of Debians Xen 4.0.1-2.
> 

Hello,

Did you find more info about this issue?

-- Pasi


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