Re: [Xen-devel] domU is causing misaligned disk writes

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Tue, Apr 20, 2010 at 01:09:58AM -0700, Tracy Reed wrote:
> Anyone know why my xen xvda devices would be doing (apparently)
> unaligned writes to my SAN causing horrible performance and massive
> seeking and lots of reading for page cache backfill? BUT writing to
> the device in the dom0 is very fast and causes no extra reads?
> 
> I am running the 2.6.18-164.11.1.el5xen xen/kernel which came with
> CentOS 5.4
> 
> After spending a lot of time banging my head on this I seem to have
> finally tracked it down to a difference between domU and dom0.  I
> never would have thought it would be this but it is extremely
> reproduceable. We're talking a difference of 4-5x in write speed.
> Reads are equally fast everywhere.
> 
> I am using AoE v72 kernel module (initiator) on a Dell R610's to talk
> to vblade-19 (target) on Dell R710's all running CentOS 5.4. I have
> striped two 7200 RPM SATA disks and exported the md with AoE (although
> I have done these tests with individual disks also). Read performance
> is excellent:
> 
> # dd of=/dev/null if=/dev/xvdg1 bs=4096 count=3000000
> 3000000+0 records in
> 3000000+0 records out
> 12288000000 bytes (12 GB) copied, 106.749 seconds, 115 MB/s
> 
> I dropped the cache with:
> 
> echo 1 > /proc/sys/vm/drop_caches
> 
> on both target and initiator before starting the test. This is great
> for just a single gig-e link. This suggests that the network is fine.
> 
> However, write performance is odious. Typically around 20MB/s. It
> should be more like 70MB/s per disk or better (7200rpm SATA) and max
> out my gig-e with write performance similar to the above read
> performance. I mentioned above that these are unaligned writes because
> when running iostat on the target machine I can see lots of reads
> happening which are surely causing seeks and killing
> performance. Typical is something like 8MB/s of reads while doing
> 16MB/s of writes.
> 
> HOWEVER, if I do the writes from the dom0 the performance is
> excellent:
> 
> # dd if=/dev/zero of=/dev/etherd/e6.2 bs=4096 count=3000000
> 3000000+0 records in
> 3000000+0 records out
> 12288000000 bytes (12 GB) copied, 104.679 seconds, 117 MB/s
> 
> And I see no reads happening on the disks being written to in
> iostat. Purely streaming writes at high speeds.
> 
> I have had AoE working very well with Xen previously although not with
> this particular hardware/xen/aoe version. Also it occurs to me that in
> the past when I have done this I network booted the domU's and they
> got root over AoE using a complicated initrd that I cooked up. In the
> last year or so I decided that it was too complicated and went to
> booting my dom0's from compact flash with the AoE driver in the dom0
> instead of the domU. I now handing the domU xvd's from the AoE driver
> in dom0. I strongly suspect that this is why things worked great
> before but stink now. Unfortunately I don't have a working network
> boot initrd setup like I used to and although I still have all of the
> code etc. it would take a while to set up. I don't want to run that
> setup in production anymore anyway if I can help it.
> 
> I have tried manually aligning the disk by setting the beginning of
> data on the partition from 63 to 64 (although this is usually done for
> RAID alignment) and I have tried changing the disk geometry to account
> for the extra partition table which causes a half-block page-cache
> misalignment as described by the ever insightful Kelsey Hudson in his
> writeup on the issue here:
> 
> http://copilotco.com/Virtualization/wiki/aoe-caching-alignment.pdf/at_download/file
> 
> All to no avail. What am I missing here? Why is domU apparently
> fudging my writes?
> 

Please paste your domU partition table:
sfdisk -d /dev/xvda

Are you using filesystems on normal partitions, or LVM in the domU? 
I'm pretty sure this is a domU partitioning problem.

-- Pasi


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