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 6:37 PM, Ross Walker <rswwalker@xxxxxxxxx> wrote:

> 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:?
Sorry, missed the virtual block device bit...

> > 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?
Never mind my comment, writes are only buffered using file: io, but  
they are buffered in the domU's page cache which is where you might be  
seeing the performance difference.
> > 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?
After re-reading I guessed you meant you attached to the iSCSI lun  
after booting into the VM not as the OS disk.
Again you are most likely seeing all cache affect and not the real io.

> > 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.
Try increasing the size of the bonnie test file to defeat the cache,  
say 2x the memory of the dom0 or domU or target which ever is largest.
-Ross


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