RE: [Xen-users] xen 3.3 -> tap:aio performance over nfs

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> I've been getting disappointing performance for file-backed disks
mounted
> using tap:aio over an NFS mount.
> For example, bonnie++ reports 3 MB/s for block writes in the domU to
> that
> disk, but in dom0, bonnie++ reports 38 MB/s for that same mount. The
> same domU can get 40 MB/s to the same filer using a phy: block device
> over iSCSI .
> 
> It seems that
> 1. tap:aio causes synchronous writes to the nfs mount (6 writes, wait
for
> ack, write 6 more etc) as opposed to NFSv3 async write
> 2. Each synchronous write writes very small blocks (4K bytes vs max of
32K
> bytes)
> 
> Is there any configuration option to tweak this behavior to get
better-
> performing async writes to the filer?
> 

I am assuming here that you have two physical servers, one with your
file-backed disk images on it, and a second which runs Xen and accesses
those files over NFS. If I got that the wrong way around then ignore the
rest of this email :)

I think that using tap:aio over NFS is not the correct solution to the
problem. I can think of a whole load of ways it could go wrong...

You might like to try file: but you would be sacrificing performance for
integrity.

But I think the correct way to do it would be to use iSCSI or AoE. Using
vblade you could set up AoE very very quickly and do some testing,
assuming that you are on the same broadcast segment (AoE can't cross a
router).

James


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