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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Thu, Mar 24, 2011 at 02:15:01PM +0100, Adi Kriegisch wrote:
> Dear Pasi,
> 
> I am still investigating this... (and I also wrote a bug report about it
> which is still waiting for an update).
> 

If you submitted it to xen bugzilla then it's also worth
sending an email to xen-devel .. bugs are mostly handled/discussed
on xen-devel mailinglist.

Thanks for the update,

-- Pasi

> > > I investigated some serious performance drop between Dom0 and DomU with
> > > LVM on top of RAID6 and blkback devices.
> [SNIP]
> > > 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 recompiled the kernel with those values hardcoded. It had no direct
> impact on the benchmark results. So this assumtion was wrong.
> 
> > > I can somehow mitigate the issue by using a way smaller chunk size but 
> > > this
> > > is IMHO just working around the issue.
> Using a smaller chunk size indeed helps to improve write speeds but read
> speeds are getting worse then.
> Making benchmarks with different chunk sizes and different kernels is quite
> time consuming; therefor I did not provide an update on that yet.
> 
> > > 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?
> I'd be really glad if someone who is using raid5 or raid6 on Dom0 could
> provide some numbers on this.
> 
> Probably this is related to the weak hardware I am using: This machine is
> an Atom D525 with 4 (hyperthreaded) cores. Maybe the issue is related to
> in-order/out-of-order execution or something like that?
> 
> > Did you find more info about this issue?
> To sum it up: no, not yet! ;-)
> 
> Thanks for asking,
>       Adi 

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