Ben, thanks for the suggestion, it didn't work, but it led me to test the network of the domu and it turns out that with netperf to a different machine I can get ~500Mbit/s from the domU and ~950Mbit/s from the dom0. >From another domU on a different dom0 (older systems, centos 5.3 with kernel 2.6.18-92.1.22.el5xen i686 instead of 2.6.18-164.el5xen x86_64) netperf produces 950Mbit/s as well. The weird thing is however that it's dom0 that acts as iSCSI initiator, so it is dom0 that is using the network and there it works fine. I'll go do some more tests. Daniel. * Ben Montanelli <montanelli at rivint.com> [09/24/2009 14:45]: > 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. > > > > _______________________________________________ > > CentOS-virt mailing list > > CentOS-virt at centos.org > > http://lists.centos.org/mailman/listinfo/centos-virt > > > > _______________________________________________ > CentOS-virt mailing list > CentOS-virt at centos.org > http://lists.centos.org/mailman/listinfo/centos-virt