On Tue, May 10, 2016 at 09:01:20PM +0000, Mooney, Sean K wrote:
Today the only native way to request Libvirt to create a vm who's memory is backed by a memdev which can be Accessed via a file descriptor is to request hugepage backed memory. This requires the operator to Manage and configure hugepage on each of their compute hosts and take special care to ensure that vms are Not placed on host were vhost-user interface are used if they do not request hugepages.
Today it is possible to use Libvirt to spawn a vm without hugepage memory and a file descriptor backed memdev Via the use of the qemu:commandline element.
<qemu:commandline> <qemu:arg value='-object'/> <qemu:arg value='memory-backend-file,id=mem,size=1024M,mem-path=/var/lib/libvirt/qemu,share=on'/> <qemu:arg value='-numa'/> <qemu:arg value='node,memdev=mem'/> <qemu:arg value='-mem-prealloc'/> </qemu:commandline>
I created a proof of concept patch to nova to demonstrate that this works however to support this usecase in Nova a new xml element is required. https://review.openstack.org/#/c/309565/1
I would like to propose the introduction of a new subelemnt to the memorybacking element to request file discrptro backed memory
<memoryBacking> <filedescriptor size_mb="1024" path="/var/lib/libvirt/qemu" prealloc="true" shared="on" /> </memoryBacking>
Specifying a size is not required - we already know how big memory must be for the guest. We already have a memAccess='shared' attribute against the <numa> element that is used to determine if the underlying memory should be setup as shared. We could define a further element that lets us control memory access mode for guests without NUMA topology specified. <memoryBacking> <access mode="shared"/> </memoryBacking> For huge pages it seems we unconditionally pass --mem-prealloc. I'm thinking we could perhaps make that configurable via an element <memoryBacking> <allocation mode="immediate|ondemand"/> </memoryBacking> to control use of -mem-prealloc or not. So all that remains is a way to request file based backing of RAM. As with huge pages, I think we should hide the actual path from the user. We should just use /dev/shm as the backing for non-hugepage RAM. For this we could define something like <memoryBacking> <source type="file|anonymous"/> </memoryBacking> Putting that all together, to get what you want we'd have <memoryBacking> <source type="file"/> <access mode="shared"/> <allocation mode="immediate"/> </memoryBacking> Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|