Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/10/2010 04:51 PM, Alexander Graf wrote:
> Thanks to recent improvements, qemu flushes guest data to disk when the guest
> tells us to do so.
>
> This is great if we care about data consistency on host disk failures. In cases
> where we don't it just creates additional overhead for no net win. One such use
> case is the building of appliances in SUSE Studio. We write the resulting images
> out of the build VM, but compress it directly afterwards. So if possible we'd
> love to keep it in RAM.
>
> This patchset introduces a new block parameter to -drive called "flush" which
> allows a user to disable flushing in odd scenarios like the above. To show the
> difference in performance this makes, I have put together a small test case.
> Inside the initrd, I call the following piece of code on a 500MB preallocated
> vmdk image:
>    

This seems like it's asking for trouble to me.  I'm not sure it's worth 
the minor performance gain.

Regards,

Anthony Liguori