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Re: [Xen-devel] 4.2.1: Poor write performance for DomU.

On 20/02/2013 8:49 PM, Steven Haigh wrote:
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

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.

Just wondering if there is any further news or tests that I might be able to do on this?

Steven Haigh

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

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