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 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. 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. Will it work for you? Den