f214978a42
When two AIO requests write to the same cluster, and this cluster is unallocated, currently both requests allocate a new cluster and the second one merges the first one when it is completed. This means an cluster allocation, a read and a cluster deallocation which cause some overhead. If we simply let the second request wait until the first one is done, we improve overall performance with AIO requests (specifially, qcow2/virtio combinations). This patch maintains a list of in-flight requests that have allocated new clusters. A second request touching the same cluster is limited so that it either doesn't touch the allocation of the first request (so it can have a non-overlapping allocation) or it waits for the first request to complete. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Read the documentation in qemu-doc.html. Fabrice Bellard.