Re: [Libguestfs] [PATCH virt-v2v] v2v: -o rhv-upload: Enable multi-conn

On Tue, Aug 03, 2021 at 10:36:47PM +0300, Nir Soffer wrote: The rules on the client is that if you want action A to be visible by B, you have to wait until the server's response to action A has been received, then send the flush, then wait for the server's response to the flush, then send action B. If the server supports multi-conn, then those rules apply regardless of which connection sends action A, the flush, or action B; if multi-conn is not relied on, then all of A, the flush, and B must be on the same connection. qemu-nbd is built on top of the qemu block layer which has its own internal serialization that guarantees that any flush action acted on by the block layer will stabilize any earlier writes and finish before any further writes - but that does NOT take into account the possibility that parallel clients can race to have out-of-order transmission effects. Even with a single client connection, where the client batch-sends A/flush/B over the wire, the qemu-nbd coroutine code is such that based on the order in which the OS wakes up semaphores in response to data being available to read from the socket, the calls into the block layer could still be rearranged as B/flush/A. That is, if an NBD client issues action A but does not wait for A's response before issuing a flush, or the client issues action B before getting the response to the flush, you cannot ensure that qemu-nbd processes those commands in the same order. So in general, the client has to actually wait for the response to the write before sending the flush, if the write is supposed to be picked up as part of the flush. But that's only when you care about the effects of action A definitely being visible to action B. There is nothing stopping a client from issuing a flush in parallel with in-flight write/zero commands; BUT that client must also be prepared that the writes that are in flight might not be flushed by the flush command. In the case of nbdcopy, we know that our writes are non-overlapping and at a large enough granularity to not trigger read-modify-write sharding, so we really don't care whether action A is visible before starting action B; all we REALLY care about is that after ALL write actions are done, we flush the disk at least once before disconnecting, so that there is no data loss due to disconnecting too early. In this style of operation, you aren't going to issue the flush until after all other commands have completed, so there are no in-flight commands in parallel with the flush to worry about in the first place. Having a flush in parallel with other in-flight operations tends to happen more with actual guest use of a disk, when the guest OS really does want to ensure that action A is visible to action B (say two cooperating processes manipulating a common file in the file system), but does not care about action C being performed in parallel by an independent process (managing a different file in the file system). -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org