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

[Ross Walker <rswwalker@xxxxxxxxx>]

On Jan 13, 2009, at 5:48 PM, "Christopher Chen" <muffaleta@xxxxxxxxx>  
wrote:
> 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.
That's kind of lopsided I'd expect it the  other way around.

Is this hardware RAID on the backend with write-back cache?

> 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.
How did you attach it, what Xen driver did you use phy: or file:?

> 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 confused.

I thought you said above you got 110MB/s on dom0 and 45MB/s on the domU?

> I'm going to give a shot doing the initialization from the DomU (just
> for kicks...)...and wow! 129MB/sec writes, 49MB/sec reads.
You've completely lost me now, what do you mean initialization? Do you  
mean boot domU off of iSCSI directly?
> 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.
Any layer you add is going to create latency.

If you can be a little more clearer I'm sure an accurate explanation  
can be made.
-Ross


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