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authorStefan Hajnoczi <stefanha@linux.vnet.ibm.com>2012-07-27 09:05:22 +0100
committerKevin Wolf <kwolf@redhat.com>2012-08-06 22:39:14 +0200
commitbfe8043e9214d2fc6572cc72b5f2218308747acd (patch)
tree40d56873d87524c6b4b062f43851e44ededcae9c /bswap.h
parent91cf8a35e777cdbdc3bf0d39c319ac9f76b7a296 (diff)
qcow2: implement lazy refcounts
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 Update qemu-iotests 031 and 036 since the extension header size changes when we add feature bit table entries. Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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