On 04/15/2011 05:21 PM, address@hidden wrote:
On 4/15/2011 10:29 AM, Christoph Hellwig wrote:
On Fri, Apr 15, 2011 at 09:23:54AM -0700, Badari Pulavarty wrote:
True. That brings up a different question - whether we are doing
enough testing on mainline QEMU :(
It seems you're clearly not doing enough testing on any qemu. Even
the RHEL6 qemu has had preadv/pwritev since the first beta.
Christoph,
When you say "you're" - you really meant RH right ? RH should have
caught this in their
regression year ago as part of their first beta. Correct ?
Unfortunately, you are picking on person who spent time find &
analyzing the regression,
narrowing the problem area and suggesting approaches to address the
issue :(
This is a pretty silly discussion to be having.
The facts are:
1) NFS sucks with preadv/pwritev and O_DIRECT -- is anyone really
surprised?
2) We could work around this in QEMU by doing something ugly
3) We have no way to detect when we no longer need a work around which
makes (2) really unappealing.
4) That leaves us with:
a) waiting for NFS to get fixed properly and just living with
worse performance on older kernels
b) having a user-tunable switch to enable bouncing
I really dislike the idea of (b) because we're stuck with it forever
and it's yet another switch for people to mistakenly depend on.
I'm still waiting to see performance data without O_DIRECT. I suspect
that using cache=writethrough will make most of this problem go away
in which case, we can just recommend that as a work around until NFS
is properly fixed.