[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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 09:19:35 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0

On 05/26/2010 09:12 AM, Aurelien Jarno wrote:
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.

It's not a functional change. It's a change in performance. There are tons of changes in performance characteristics of qemu from version to version. It's not even a massive one.

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.

This isn't a feature. This is a change in performance. No one is not able to satisfy their use case from this behavior.

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

I highly doubt that this is even visible on benchmarks without using KVM. The improvement on a microbenchmark was relatively small and the cost from TCG would almost certainly dwarf it.

Also, remember before KVM, we had single threaded IO and posix-aio (which is still single threaded). If KVM never happened, block performance would be far, far worse than it is today with cache=writeback.


Anthony Liguori

reply via email to

[Prev in Thread] Current Thread [Next in Thread]