Re: [Xen-users] overhead of Xen when using iSCSI

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

Siim Vahtre wrote:
> I have setup iSCSI targets (with open-iscsi) to dom0 (Xen 3.1), which 
> are exported to paravirtualized guests:
> disk = [ 'phy:/dev/iscsi/test0,hda1,w' ]
>
> Everything works fine, but there is huge overhead. If I write to iSCSI 
> device in dom0 I get write speeds of about 29Mbytes/s, but if I write 
> to /dev/hda1 in domU I only get about 14Mbytes/s.
> This kind of performance drop does not happen with regular disks or 
> even NBD - they have some overhead but not 50% like I have with iSCSI.
> I have searched the list and I know that some people have successfully 
> run setup similar to mine, so I'd like to ask if anyone could please 
> share their experience and tell me if 50% performance drop is really 
> normal.
> Thanks!
The performance hit happens for me with local disk on Dom0 and with 
dedicated partitions for DomU use, and it's even worse with file images 
on Dom0. I haven't made or had the chance to use iscsi yet, since I only 
recently laid my hands on a GigE switch for my setup.
How did you test this? And do you have good guidelines on setting up 
iscsi? I'm finding fairly useless directions all over the place, but no 
good guides for RedHat/CentOS setups.
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