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):
virt-manager-0.8.3-3.fc13.noarch
libvirt-0.7.7-3.fc13.x86_64


How reproducible:
Always.

Steps to Reproduce:
1. Go to disk properties.

  
Actual results:
No dropdown list for cache mode.


Expected results:
Cache mode: dropdown list with options "writethrough (default)", "writeback", "none (Direct_IO)".
Nb. I'm not sure if "none" really equals dio.


Additional info:

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
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='writeback'/>
      <source file='/mnt/storage_ST_250G/vms/scratch-F13-32b.img'/>
      <target dev='vda' bus='virtio'/>

the "cache='writeback'" part was added manually.