Re: [libvirt] [RFC] finegrained disk driver options control

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.