Re: [Xen-users] Xen Disk I/O performance vs native performance

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Can you guys tell me how much slower your Disk I/O performance is using
> Xen, compared to native performance ?
>
> On my RAID 5 machine, using Ubuntu feisty's
> linux-ubuntu-modules-2.6.22-14-xen kernel, xen 3.1, LVM disks, read and
> writes to the disk are something between 2 and 3 times slower.
>
> Is that considered normal, or especially bad ? If it's worse than the
> average, what kind of information do you want me to give you to diagnose
> the problem ?

That's lower than I expected.  Can you fill in a few more details about the 
precise nature of your setup?  The following questions are a starting point:

1) Is this comparing Xen dom0 with native Linux running on the same 
partition / LVM volume on the same machine?
2) Are your numbers from the domU or from dom0?
3) How are the RAID 5 and LVM implemented?  RAID 5 in hardware or software?  
LVM in dom0 or domU (or both)?
4) Any interesting messages in dmesg that might suggest that the hardware 
support is not working correctly?
5) Any other ideas?

Another thing that you could try is to download VMKNOPPIX and boot into Xen 
off that, then try doing some runs mounting your disk and reading / writing 
it using that.  You might need to rerun this a few times to ensure that 
everything is paged in off the CD-ROM drive, otherwise that might affect the 
results badly.  I assume VMKNOPPIX uses something closer to the standard 
XenLinux kernel, so this might help to eliminate anything Ubuntu's packaging 
or choice of kernel version may have introduced.

Cheers,
Mark


-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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