Re: 4.2.1: Poor write performance for DomU.

[Steven Haigh <netwiz@crc.id.au>]

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On 20/02/2013 7:49 PM, Steven Haigh wrote:
> On 20/02/2013 7:26 PM, Roger Pau Monné wrote:
>> On 20/02/13 03:10, Steven Haigh wrote:
>>> Hi guys,
>>>
>>> Firstly, please CC me in to any replies as I'm not a subscriber these
>>> days.
>>>
>>> I've been trying to debug a problem with Xen 4.2.1 where I am unable to
>>> achieve more than ~50Mb/sec sustained sequential write to a disk. The
>>> DomU is configured as such:
>>
>> Since you mention 4.2.1 explicitly, is this a performance regression
>> from previous versions? (4.2.0 or the 4.1 branch)
>
> This is actually a very good question. I've reinstalled my older
> packages of Xen 4.1.3 back on the system. Rebooting into the new
> hypervisor, then starting the single DomU again. Ran bonnie++ again on
> the DomU:
>
> Still around 50Mb/sec - so this doesn't seem to be a regression, but
> something else?

I've actually done a bit of thinking about this... A recent thread on 
linux-raid kernel mailing list about Xen and DomU throughput made me 
revisit my setup. I know I used to be able to saturate GigE both ways 
(send and receive) to the samba share served by this DomU. This would 
mean I'd get at least 90-100Mbyte/sec. What exact config and kernel/xen 
versions this was as this point in time I cannot say.

As such, I had a bit of a play and recreated my RAID6 with 64Kb chunk 
size. This seemed to make rebuild/resync speeds way worse - so I 
reverted to 128Kb chunk size.

The benchmarks I am getting from the Dom0 is about what I'd expect - but 
I wouldn't expect to lose 130Mb/sec write speed to the phy:/ pass 
through of the LV.

 From my known config where I could saturate the GigE connection, I have 
changed from kernel 2.6.32 (Jeremy's git repo) to the latest vanilla 
kernels - currently 3.7.9.

My build of Xen 4.2.1 also has all of the recent security advisories 
patched as well. Although it is interesting to note that downgrading to 
Xen 4.1.2 made no difference to write speeds.

-- 
Steven Haigh

Email: netwiz@crc.id.au
Web: http://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897
Fax: (03) 8338 0299


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