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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 17:40:35 +0200 |
User-agent: |
Mozilla-Thunderbird 2.0.0.22 (X11/20090707) |
Anthony Liguori a écrit :
> 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.
It is something clearly visible. Before fsync() was not used, and it
happens this syscall can be very expensive (ie a few seconds, especially
with some other i/o load on the system) on ext3 with not so old kernels.
A google search for "firefox fsync" will give you a few pointers.
> 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.
>
io thread is not enable by default in QEMU.
--
Aurelien Jarno GPG: 1024D/F1BCDB73
address@hidden http://www.aurel32.net
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, (continued)
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Aurelien Jarno, 2010/05/26
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Aurelien Jarno, 2010/05/26
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Kevin Wolf, 2010/05/26
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Anthony Liguori, 2010/05/26
- [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Paolo Bonzini, 2010/05/26
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Aurelien Jarno, 2010/05/26
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Anthony Liguori, 2010/05/26
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive,
Aurelien Jarno <=
- Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Anthony Liguori, 2010/05/26
Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Avi Kivity, 2010/05/26
[Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive, Anthony Liguori, 2010/05/17