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Re: [Libguestfs] [PATCH 4/6] v2v: rhv-upload-plugin: Support multiple connections

On Tue, Jan 26, 2021 at 08:51:59AM -0600, Eric Blake wrote:
> 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.

It happens that nbdcopy does call nbd_flush on each handle.  (A case
of being "right but for the wrong reasons").  However we don't do
multi-conn unless the server advertises it, and in light of what you
say above perhaps we should.

> >   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."

Although the text only mentions flush & FUA, I wonder if there's
another case where multi-conn should not be advertised or used: That
is where the block or sector size is larger than the size of writes,
so writes are being turned into r/m/w cycles.  Multiple adjacent (even
non-overlapping) writes could then interfere with each other.

This is vanishingly unlikely to affect nbdcopy which uses a huge block
size, but would it be a problem in the NBD protocol itself?


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