Re: [Xen-users] Performance of network block devices (iSCSI)

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Longina Przybyszewska wrote:

> > OK, I've *finally* managed to scrounge another box to do some 
> > testing on. I've setup open-iscsi in Dom0 and performance seems to 
> > be a lot better - I'm getting about 20MB/s and the backup server 
> > isn't sat on 99% wio :-)
> 1. I wonder what test did you make to check performance?
> I have the same concept with iscsi initiator directly on the domU 
> via bonding interface-->dedicated vlan on top of 
> bonding-->transparent bridge in Dom0-->DomU's eth1.
> I would expect more advantages (performance, administration) with 
> iscsi-->DomU model.
I found that having the iSCSI initiator on DomU gave "poor" 
performance (check the start of this thread). It's been suggested 
that the single threaded model of Dom0 networking may be to blame. 
Having the iSCSI on the DomU was my preferred setup as that would 
allow me to shut down a guest, move it's config and storage to 
another host, start it up and it would be just the same.
Performance was tested rather crudely with :

time ( dd if=/dev/zero of=testfile bs=1024k count=2048 ; sync )

this writes a file with 2GB of zeros, chosen to be larger than memory 
available, and the sync makes sure we include the time taken to write 
what does fit in memory.
> > I assume this is because the device name can change across 
> > restarts. Since I'm not mounting the volume in Dom0, just passing 
> > it to the guest in a "disk = [ 'phy:v ..." line, any suggestions on 
> > the best way to deal with this ?
> 2. how can you pass the storage without mounting it/having access to, in Dom0?
Dom0 does have 'access' since it appears as a scsi drive (eg sdc, 
with a partition sdc1). It is NOT mounted in Dom0, the device 
(/dev/sdc1, or /dev/disk/by-id/...) is given to the DomU in the 
"disk=" line. The DomU mounts it just the same as the rest of the 
volumes given to it.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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