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Re: [Qemu-devel] Default cache mode
From: |
Christoph Hellwig |
Subject: |
Re: [Qemu-devel] Default cache mode |
Date: |
Wed, 29 Jun 2011 15:55:45 +0200 |
User-agent: |
Mutt/1.5.17 (2007-11-01) |
On Wed, Jun 29, 2011 at 07:08:18AM -0500, Anthony Liguori wrote:
> As long as we advertise wce and wce can be toggled from the guest, I don't
> think the default is all that important. I think cache=on is the right
> default for most common use cases.
What do you mean with cache=on? We have
cache=[none,writeback,writethrough,unsafe]. As discussed about a dozen
times before these are an incomplete choice of the [odirect=on,off;
osync=on,off, ignore cache flushes=on,off] matrix.
Not using O_DIRECT when it is available is almost always the wrong choice.
It means an additional data copy, VM pressure and exercise of often suck
fsync/O_SYNC codepatheses, with only a benefit if you have multiple VMs
actually using the same dataset with the same COW backing image.
Using WCE = 0 or 1 is a tradeoff of safety vs performance. WCE=0 performance
generally sucks badly, but offers data integrity for older or non-PCish host
operating systems.
Re: [Qemu-devel] Default cache mode, Christoph Hellwig, 2011/06/29
Re: [Qemu-devel] Default cache mode, Anthony Liguori, 2011/06/29
- Re: [Qemu-devel] Default cache mode,
Christoph Hellwig <=
Re: [Qemu-devel] Default cache mode, Alexander Graf, 2011/06/29
Re: [Qemu-devel] Default cache mode, Stefan Hajnoczi, 2011/06/29
Re: [Qemu-devel] Default cache mode, Avi Kivity, 2011/06/29