Red Hat Bugzilla – Bug 623188
Defaults to using cache with an LVM backed storage.
Last modified: 2013-10-02 16:12:51 EDT
Description of problem:
Defaults to using cache with an LVM backed storage. The use of caching with raw partitions (LVM) results in significantly lower performance than no cache at all.
Steps to Reproduce:
1. Create a new VM using LVM backed storage
Cache is enabled for the VM's disks residing within LVM.
Cache should be disabled for disks residing within LVM.
QEMU also supports a wide variety of caching modes. Writeback is useful for testing but does not offer storage guarantees. Writethrough (the default) is safer, and relies on the host cache. If you're using raw volumes or partitions, it is best to avoid the cache completely, which reduces data copies and bus traffic:
qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,cache=none,if=virtio
This has also been reported with Ubuntu at: https://bugs.launchpad.net/ubuntu/+source/virt-manager/+bug/568445
Choice of caching mode is a policy decision. These belong in virt-manager or other apps using libvirt.
AFAIK, this is the place to post feature requests for virt-manager, at least this is where their website directed me. Intentionally selecting a default mode that results in very poor performance (about 1/5 less) when the upstream for the virtualization engine (qemu/kvm) clearly indicates that another mode is preferable is (IMHO) a bad choice. Furthermore, from what I can tell, virt-manager doesn't appear to provide any means of changing or overriding the default. A user must instead manually edit the server's XML definition of the VM in question.
Reopening against virt-manager as recommended on mailing list.
Fixed upstream now