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.

> 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.