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Re: [Xen-users] Severe slow disk write issue on LVM-deployed DomU

On 01/08/13 12:56, g wrote:
Hi everyone.
I have a really severe performance issue on disk write on my domU's PV guest.
- Xen 4.1 fresh install on Debian Wheezy (kernel 3.2.0-4-amd64) via apt-get
- Simple disk (no raid) for Linux/Xen Dom0 installation
- LVM2 on top of raid5 disk raid5 disk for storing DomU
- Single LV for each DomU

The problem
While write speed (mesured via dd command) on Dom0 is near 163MB/s on the mounted DomU LV , the same dd command provides 8MB/s if executed within
the running DomU (with "noop" I/O scheduler set)
Running "top" in DomU, i  get high load average due to high %wa (no high CPU load)

DomU disk is submitted to Xen via configuration file with:
disk  = [ 'phy:/dev/xenstore/test,xvda1,w' ]

If the same DomU is deployed on file image ("test.img" cloned from "test" LV), write speed rise up to 63MB/s
disk  = [ 'file:/home/test.img,xvda1,w' ]

Obviously I can provide further detailed configuration parameters

Does anyone faced to the same problem? Does anyone can point me in the right direction?

A couple of things to check.

shutdown the VM, and check the read/write speed on the LV from the dom0 /dev/xenstore/test

Most likely, you have one or more snapshots of the LV, and this totally kills performance. You might get away with a single snapshot and performance might still be acceptable.

Also, try adjusting the value of /sys/block/md1/md/stripe_cache_size
I ended up using 4096 which seemed to provide the best performance (I'm using 4 x SSD in RAID5).

Verify performance on dom0 first, then compare it to the performance within the domu.
Make sure you allocate sufficient CPU resources to dom0 as well, or performance of the domu can suffer.


Adam Goryachev Website Managers www.websitemanagers.com.au
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