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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 09:08:24 -0500
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On 05/26/2010 09:03 AM, Kevin Wolf wrote:
Am 26.05.2010 15:42, schrieb Anthony Liguori:
On 05/26/2010 03:43 AM, Kevin Wolf wrote:
Am 26.05.2010 03:31, schrieb Anthony Liguori:

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

We know better what you stupid user want?
What percentage of qemu users do you think have actually read qemu-doc.texi?
As I said, put the warning in the option name like cache=unsafe or
something even more scary and I'm all for it.

It's not a stretch for someone to have heard that cache options can
improve performance, and then see cache=volatile in the help output, try
it, and then start using it because they observe a performance improvement.

That's not being stupid.  I think it's a reasonable expectation for a
user to have that their data is safe.
You seem to think that the user is too stupid to allow him to use this
option even if he's perfectly aware what it's doing. It's a useful
option if it's used right.

No, that's not what I said. I'm saying we need to try hard to make a user aware of what they're doing.

If it spit out a warning on stdio, I wouldn't think a compile option is needed. Even with help output text, I'm concerned that someone is going to find a bad example on the internet.

cache=unsafe addresses the problem although I think it's a bit hokey.

We need to make clear that it's dangerous when it's used in the wrong
cases (for example by naming), but just disabling is not a solution for
that. You don't suggest that "no sane distribution" should ship rm,
because it's dangerous if you use it wrong, do you?

You realize that quite a lot of distributions carry a patch to rm that prevents a user from doing rm -rf /?


Anthony Liguori


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