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

[Anthony Liguori <anthony@codemonkey.ws>]

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.

Regards,

Anthony Liguori