On Thu, Mar 16, 2017 at 08:31:08PM +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 > > > > 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. Ignoring the question of generic option passthrough, I think we can model the cache settings in libvirt XML explicitly. Other types of disk besides qcow2 can have a cache concept, so I think we could create something like this: <driver name='qemu' type='qcow2' ....> <cache> <clean interval="2" unit="seconds"/> <bank name="l2" size="1024" unit="KiB"/> <bank name="refcount" size="1024" unit="KiB"/> </cache> </driver> The "bank" element would be permitted to be repeated multiple times if a particular diskk driver had multiple caches it needed. In the storage vol XML, we would want a way top report what the size of the L2 and refcount tables are when reporting qcow2 volumes, so apps know the maximum sensible size to use for cache. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
