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

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 > > Regards, > Daniel In general this policy means that the management software which wants to implement some differentiation in between VMs f.e. in disk tuning is forced to use qemu:commandline backdoor. That is a pity. Exactly like in the case with additional logs. Thank you for the discussion. At least I have found new way to perform some fine tuning.