Re: [Qemu-discuss] Disk Performance

[Stephan von Krawczynski]

On Fri, 30 May 2014 11:29:08 -0400
address@hidden (Dale R. Worley) wrote:

> > From: Stephan von Krawczynski <address@hidden>
> 
> > let me shortly tell you my own experiences with virtual disk performance.
> > In short: I stopped using image files of any kind. I switched to lvm
> > partitions. The reason is that your virtual hosts will choke if you copy 
> > files
> > to your physical host over the net or on the host itself. I tried everything
> > to prevent that but in the end I gave up.
> 
> I'm no expert on this, but it sounds to me like the file copy is using
> memory to cache disk blocks, swapping out the memory that is the
> emulated memory of the guests.  It seems to me that the solution to
> that is to force the Qemu process's memory space to remain in RAM.
> 
> Is there a way to do that in Linux?

No.

Nevertheless the whole story is not as simple as it seems here. There is no
good reason why the cache is damaged for an open image file on a box that has
more RAM than all operations summed up require.
In detail the problem seems not to be the cache, but specifically the cache
flushing (syncing of dirty cache pages). Whatever is configured (quick, small
flushes or late big ones) damages the qemu performance dramatically.
Contrary to that the use of lvm partitions is not affected at all.

Regards,
Stephan