On 03/16/2017 06:52 PM, Daniel P. Berrange wrote: > 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 > > > Regards, > Daniel Funny thing that this information could come from the outside world, f.e. from the SLA which is dependent from the amount of money the end-user is paying to the hosting provider. Den -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
