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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [PATCH] raw-posix: Linearize direct I/O on Linux NFS |
Date: | Fri, 15 Apr 2011 18:00:16 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.15) Gecko/20110411 Lightning/1.0b2 Thunderbird/3.1.9 |
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 theirregression 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 ugly3) 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 bouncingI 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.
Regards, Anthony Liguori
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