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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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| .. | ||
| qemu-iotests | ||
| tcg | ||
| .gitignore | ||
| check-block.sh | ||
| check-qdict.c | ||
| check-qfloat.c | ||
| check-qint.c | ||
| check-qjson.c | ||
| check-qlist.c | ||
| check-qstring.c | ||
| fdc-test.c | ||
| hd-geo-test.c | ||
| libqtest.c | ||
| libqtest.h | ||
| m48t59-test.c | ||
| Makefile | ||
| qemu-iotests-quick.sh | ||
| rtc-test.c | ||
| test-coroutine.c | ||
| test-iov.c | ||
| test-qmp-commands.c | ||
| test-qmp-input-strict.c | ||
| test-qmp-input-visitor.c | ||
| test-qmp-output-visitor.c | ||
| test-string-input-visitor.c | ||
| test-string-output-visitor.c | ||
| test-visitor-serialization.c | ||