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

From: Aurelien Jarno
Subject: Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive
Date: Wed, 26 May 2010 16:12:39 +0200
User-agent: Mozilla-Thunderbird (X11/20090707)

Anthony Liguori a écrit :
> 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.

For people who don't care about safety it's still a regression. And it
is a common usage of QEMU.

> 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.

Which basically means those distro don't care about some use cases of
QEMU, that were for most of them the original uses cases. It's sad.

Sometimes I really whishes that KVM never tried to reintegrate code into
QEMU, it doesn't bring only good things.

Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net

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