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

On Thu, Mar 16, 2017 at 04:35:36PM +0100, Kevin Wolf wrote: > Am 16.03.2017 um 16:08 hat Daniel P. Berrange geschrieben: > > 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. > > A tradeoff between memory usage and performance is policy, and setting > policy is the management layer's job, no qemu's. We can try to provide > good defaults, but they are meant for manual users of qemu. libvirt is > expected to configure everything exactly as it wants it instead of > relying on defaults. The question though is how is an app supposed to figure out what the optimal setting for cache size is ? It seems to require knowledge of the level of disk fragmentation and guest I/O patterns, neither of which are things we can know upfront. Which means any atttempt to set cache size is little more than ill-informed guesswork