Description of problem:
On some filesystems (btrfs) default caching in qemu makes guest extremely slow. It could be improved greatly by changing caching to writeback mode. The cost of this change: degraded reliability in case of host crash.
Please provide drop-down list to change the mode in Disk details view in virt-manager.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Go to disk properties.
No dropdown list for cache mode.
Cache mode: dropdown list with options "writethrough (default)", "writeback", "none (Direct_IO)".
Nb. I'm not sure if "none" really equals dio.
It is possible to manually edit guest .xml file to change caching mode. This change is retained by virt-manager and provides requested performance. Example:
# grep -C2 cache /etc/libvirt/qemu/scratch-F13-32b.xml
<disk type='file' device='disk'>
<driver name='qemu' type='raw' cache='writeback'/>
<target dev='vda' bus='virtio'/>
the "cache='writeback'" part was added manually.
This is upstream now:
Indeed, virt-manager in F14 contains cache mode selector.