[Xen-users] iSCSI initiator on Dom0, exported to DomU via xvd, Disk IO Drops in Half...

["Christopher Chen" <muffaleta@xxxxxxxxx>]

Hi there!

I've been wrestling with an issue for a little bit now--

In my test environment, I have tgtd running on a Centos 5.2 box, with
a raid 10 array backing it.

The initiators are also Centos 5.2 boxes running Xen 3.0.3 userland
with a Xen 3.1.2/Linux 2.6.18 kernel (as from repos).

Bonnie++ on the Dom0 shows about 110MB/sec writes, and 45MB/sec reads.

I've attached the iSCSI LUN to the DomU as a virtual block device, and
I'm seeing 47MB/sec writes, and 39MB/sec reads.

I've tried a few things, like running against a local disk, and
suprisingly, writes on the DomU are faster than the Dom0--can I assume
the writes are buffered by the Dom0.

I'm going to give a shot doing the initialization from the DomU (just
for kicks...)...and wow! 129MB/sec writes, 49MB/sec reads.

This is all with bonnie++ -d /mnt -f -u root:root

Anyone seen this, or have any ideas?

Is any additional latency provided by the xen virtual block device
causing a degradation in TCP performance (i.e. a window size or
delayed ACK problem) or is the buffering also causing pain? I'm going
to keep looking, but I thought I'd ask all of you.

Cheers

cc

-- 
Chris Chen <muffaleta@xxxxxxxxx>
"I want the kind of six pack you can't drink."
-- Micah

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