Am 22.06.2012 17:08, schrieb Stefan Hajnoczi: > Lazy refcounts is a performance optimization for qcow2 that postpones > refcount metadata updates and instead marks the image dirty. In the > case of crash or power failure the image will be left in a dirty state > and repaired next time it is opened. > > Reducing metadata I/O is important for cache=writethrough and > cache=directsync because these modes guarantee that data is on disk > after each write (hence we cannot take advantage of caching updates in > RAM). Refcount metadata is not needed for guest->file block address > translation and therefore does not need to be on-disk at the time of > write completion - this is the motivation behind the lazy refcount > optimization. > > The lazy refcount optimization must be enabled at image creation time: > > qemu-img create -f qcow2 -o compat=1.1,lazy_refcounts=on a.qcow2 10G > qemu-system-x86_64 -drive if=virtio,file=a.qcow2,cache=writethrough > > Signed-off-by: Stefan Hajnoczi <[email protected]>
This one requires a feature table entry as well. Kevin
