[CentOS-virt] xen & iSCSI

Thu Sep 24 18:45:30 UTC 2009
Ben Montanelli <montanelli at rivint.com>

Are you using separate physical NICs for dom0 (isolated) and the domUs?

In a CentOS domU I just, a few hours ago, had same issue on network 
reads, but doing:

# ethtool -K eth0 tx off

then rebooted domU and it cleared up and got much closer to max. Yes, 
that is the TX checksum fix, but it seemed to affect my reads (IO in).

Daniel De Marco wrote:
> Hi,
> I'm setting up xen (on centos 5.3) and I'm facing a weird performance
> issue with iSCSI.
> I attach the iSCSI targets to dom0 and if I use them from dom0 I obtain
> good performance with simple dd tests (~100MB/s both reading and
> writing).
> I then use the block devices for the domU and if I repeat the same dd
> test
> from within the domU the write performace is still good (~100MB/s), but
> the read performance is cut in half (~55MB/s).
> I tried changing several parameters like read_ahead and such, but I can
> not obtain a good read performance in the domU.
> Any idea?
> Thanks, Daniel.
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