[CentOS-virt] xen & iSCSI

Thu Sep 24 21:01:53 UTC 2009
Ben Montanelli <montanelli at rivint.com>

I think that is an interesting coincidence about x86_64 vs. 32bit 
kernel. In this same CentOS pv domU I was talking about I created an 
x86_64 by "accident." I ended up knocking it down and going to i386 tree 
(its a web app) and found it to be snappier.

Please post your results especially if it is a kernel difference.

Daniel De Marco wrote:
> 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.
>>>
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