On 1/23/21 12:38 AM, Richard W.M. Jones wrote:
Sending flush request is more tricky; on imageio side, we have one qemu-nbd server, with multiple connections. I'm not sure if sending one flush command on one of the connections is good enough to flush all commands, so we send flush command on all connections in the flush callback.
I know the answer to this! It depends if the NBD server advertises multi-conn or not. With libnbd you can find out by querying nbd_can_multi_conn on any of the connections (if the server is behaving, the answer should be identical for any connection with the same exportname). See:
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md
"bit 8, NBD_FLAG_CAN_MULTI_CONN: Indicates that the server operates entirely without cache, or that the cache it uses is shared among all connections to the given device. In particular, if this flag is present, then the effects of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA MUST be visible across all connections when the server sends its reply to that command to the client. In the absence of this flag, clients SHOULD NOT multiplex their commands over more than one connection to the export."
For unclear reasons qemu-nbd only advertises multi-conn for r/o connections, assuming my reading of the code is correct. For nbdkit we went through the plugins a long time ago and made them advertise (or not) multi-conn as appropriate.
For qemu-nbd, supporting FLAG_CAN_MULTI_CONN on a writable connection would require work to audit that a flush request on one connection properly affects all pending writes on other connections, as visible by all subsequent reads from any of the connections. This is not a trivial task. However, that's for the general case (when multiple writers are sending requests that can overlap one another, and when you have to worry about read consistency). But when copying an image, you have a special case: each writer is only touching distinct subsets of the overall image, and you are not reading from the image. As long as those subsets do not overlap, and you do not trigger read-modify-write actions, you really don't care whether flushes are consistent between writers, because there are no consistency issues in the first place. So we _might_ be able to optimize nbdcopy for the case of parallel writes even when the NBD server does not advertise CAN_MULTI_CONN; the writers can either use FLAG_FUA for every write, or _each_ write without flushes and perform a final NBD_CMD_FLUSH; as long as EACH writer performs a final flush before disconnecting (rather than trying to optimize to just one writer flushing), you will be guaranteed that the entire copied image is now consistent, even though you could not guarantee flush consistency during the parallel writes. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org