[libvirt] [RFC] finegrained disk driver options control

[Daniel P. Berrange]

On Thu, Mar 16, 2017 at 06:00:46PM +0300, Denis V. Lunev wrote:
> On 03/16/2017 05:45 PM, Daniel P. Berrange wrote:
> > On Thu, Mar 16, 2017 at 05:08:57PM +0300, Denis V. Lunev wrote:
> >> Hello, All!
> >>
> >> There is a problem in the current libvirt implementation. domain.xml
> >> allows to specify only basic set of options, especially in the case
> >> of QEMU, when there are really a lot of tweaks in format drivers.
> >> Most likely these options will never be supported in a good way
> >> in libvirt as recognizable entities.
> >>
> >> Right now in order to debug libvirt QEMU VM in production I am using
> >> very strange approach:
> >> - disk section of domain XML is removed
> >> - exact command line options to start the disk are specified at the end
> >>   of domain.xml whithin <qemu:commandline> as described by Stefan
> >>  
> >> http://blog.vmsplice.net/2011/04/how-to-pass-qemu-command-line-options.html
> >>
> >> The problem is that when debug is finished and viable combinations of
> >> options is found I can not drop VM in such state in the production. This
> >> is the pain and problem. For example, I have spend 3 days with the
> >> VM of one customer which blames us for slow IO in the guest. I have
> >> found very good combination of non-standard options which increases
> >> disk performance 5 times (not 5%). Currently I can not put this combination
> >> in the production as libvirt does not see the disk.
> >>
> >> I propose to do very simple thing, may be I am not the first one here,
> >> but it would be nice to allow to pass arbitrary option to the QEMU
> >> command line. This could be done in a very generic way if we will
> >> allow to specify additional options inside <driver> section like this:
> >>
> >>     <disk type='file' device='disk'>
> >>       <driver name='qemu' type='qcow2' cache='none' io='native'
> >> iothread='1'>
> >>           <option name='l2-cache-size' value='64M/>
> >>           <option name='cache-clean-interval' value='32'/>
> >>       </driver>
> >>       <source file='/var/lib/libvirt/images/rhel7.qcow2'/>
> >>       <target dev='sda' bus='scsi'/>
> >>       <address type='drive' controller='0' bus='0' target='0' unit='0'/>
> >>     </disk>
> >>
> >> and so on. The meaning (at least for QEMU) is quite simple -
> >> these options will just be added to the end of the -drive command
> >> line. The meaning for other drivers should be the same and I
> >> think that there are ways to pass generic options in them.
> > It is a general policy that we do *not* do generic option passthrough
> > in this kind of manner. We always want to represent concepts explicitly
> > with named attributes, so that if 2 hypervisors support the same concept
> > we can map it the same way in the XML
>
> OK. How could I change L2 cache size for QCOW2 image?
> 
> For 1 Tb disk, fragmented in guest, the performance loss is
> around 10 times. 10 TIMES. 1000%. The customer could not
> wait until proper fix in the next QEMU release especially
> if we are able to provide the kludge specifically for him.

We can explicitly allow L2 cache size set in the XML but that
is a pretty poor solution to the problem IMHO, as the mgmt
application has no apriori knowledge of whether a particular
cache size is going to be right for a particular QCow2 image.

For a sustainable solution, IMHO this really needs to be fixed
in QEMU so it has either a more appropriate default, or if a
single default is not possible, have QEMU auto-tune its cache
size dynamically to suit the characteristics of the qcow2 image.

> There is an option <qemu:commandline> which specifically
> works like this. It is enabled specifically with changed scheme.
> OK, we can have this option enabled only under the same
> condition. But we have to have a way to solve the problem
> at the moment. Not in 3 month of painful dances within
> the driver. May be with limitations line increased memory
> footprint, but still.

Sure, you can use <qemu:commandline> passthrough - that is the explicit
temporary workaround - we don't provide any guarantee that your guest
won't break when upgrading either libvirt or QEMU though, hence we
mark it as tainted.

Regards,
Daniel
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