This series do standard basic things: - it creates intermediate buffer for all writes from QEMU migration code to QCOW2 image, - this buffer is sent to disk asynchronously, allowing several writes to run in parallel.
In general, migration code is fantastically inefficent (by observation), buffers are not aligned and sent with arbitrary pieces, a lot of time less than 100 bytes at a chunk, which results in read-modify-write operations with non-cached operations. It should also be noted that all operations are performed into unallocated image blocks, which also suffer due to partial writes to such new clusters. This patch series is an implementation of idea discussed in the RFC posted by Denis https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg01925.html Results with this series over NVME are better than original code original rfc this cached: 1.79s 2.38s 1.27s non-cached: 3.29s 1.31s 0.81s Signed-off-by: Denis V. Lunev <d...@openvz.org> CC: Kevin Wolf <kw...@redhat.com> CC: Max Reitz <mre...@redhat.com> CC: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> CC: Denis Plotnikov <dplotni...@virtuozzo.com>