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Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive
Date: Wed, 26 May 2010 08:48:14 -0500
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On 05/26/2010 03:52 AM, Aurelien Jarno wrote:
On Tue, May 25, 2010 at 08:31:20PM -0500, Anthony Liguori wrote:
On 05/25/2010 04:01 PM, Aurelien Jarno wrote:
I really think this patch can be useful, in my own case when testing
debian-installer (I already cache=writeback). In short all that is about
developing and testing, as opposed to run a VM in production, can
benefit about that. This was one of the original use case of QEMU before
KVM arrived.

Unless someone can convince me not to do it, I seriously considering
applying this patch.
There really needs to be an indication in the --help output of what
the ramifications of this option are, in the very least.  It should
That's indeed something than can be done, but to avoid double standards,
it should also be done for other features that can lead to data
corruption. I am talking for example on the qcow format, which is not
really supported anymore.

I agree.

also be removable via a ./configure option because no sane
distribution should enable this for end users.

I totally disagree. All the examples I have given apply to qemu *users*,
not qemu developers. They surely don't want to recompile qemu for their
usage. Note also that what is proposed in this patch was the default not
too long ago, and that a lot of users complained about the new default
for their usage, they see it as a regression. We even had to put a note
explaining that in the Debian package to avoid to many bug reports.
cache=writeback only answer partially to this use case.

It's hard for me to consider this a performance regression because ultimately, you're getting greater than bare metal performance (because of extremely aggressive caching). It might be a regression from the previous performance, but that was at the cost of safety.

We might get 100 bug reports about this "regression" but they concern much less than 1 bug report of image corruption because of power failure/host crash. A reputation of being unsafe is very difficult to get rid of and is something that I hear concerns about frequently.

I'm not suggesting that the compile option should be disabled by default upstream. But the option should be there for distributions because I hope that any enterprise distro disables it.


Anthony Liguori

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