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xen-users
RE: [Xen-users] Performance of network block devices (iSCSI)
>
> I have a 'backup' server to which I have a number of machines dumping
> their filesystems using rdiff-backup. The backup server is storing
> this data on a volume mounted off an iSCSI store (Dell/EMC AX100i).
> I've found the performance to be 'very poor' and asked on the
> rdiff-backup list, a response I got was :
>
> >I found that the network and I/O scheduler in Xen was a single
> >pipeline and contention was terrible. We got terrible performance
> >when we used network block devices with Xen, as the VMs would just
> >sit in waitI/O all the time when accessing the network block devices
> >(we tried AoE, NBD, iSCSI).
> >...
> >We ended up moving to OpenVZ and haven't looked back.
>
> I've done a test after copying the store to a local disk (xvda) which
> is another volume in the LVM setup of the Xen host - it's notable
> that copying the backup off the iSCSI volume ran at only about
> 1/2G/hr. The difference is quite dramatic, a backup from one client
> takes 36s to a local disk, but 9 1/2 minutes to the iSCSI box -
> that's a 15 fold difference.
>
> While copying to or from the iSCSI volume the backup server sits at
> 100% (occasionally 99%) wait-io, while backing up to the virtual disk
> it shows the normal levels of processor activity I would expect (with
> minimal wait-io).
>
> Systems are Debian Lenny, running on a Dell 2650 with hardware raid
> (PERC) and plenty of RAM.
>
> Is there something I've missed ? Is there anything I can do ?
>
Try turning off all network offloading (ethtool -k ...) in DomU. If that
doesn't improve things, try turning it off on the vif in Dom0, and then
if that doesn't work either, the hardware adapter in Dom0.
James
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