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The reason that NBD added structured reply in the first place was to allow for efficient reads of sparse files, by allowing the reply to include chunks to quickly communicate holes to the client without sending lots of zeroes over the wire. Time to implement this in the server; our client can already read such data. We can only skip holes insofar as the block layer can query them; and only if the client is okay with a fragmented request (if a client requests NBD_CMD_FLAG_DF and the entire read is a hole, we could technically return a single NBD_REPLY_TYPE_OFFSET_HOLE, but that's a fringe case not worth catering to here). Sadly, the control flow is a bit wonkier than I would have preferred, but it was minimally invasive to have a split in the action between a fragmented read (handled directly where we recognize NBD_CMD_READ with the right conditions, and sending multiple chunks) vs. a single read (handled at the end of nbd_trip, for both simple and structured replies, when we know there is only one thing being read). Likewise, I didn't make any effort to optimize the final chunk of a fragmented read to set the NBD_REPLY_FLAG_DONE, but unconditionally send that as a separate NBD_REPLY_TYPE_NONE. Signed-off-by: Eric Blake <eblake@redhat.com> Message-Id: <20171107030912.23930-2-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> |
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| client.c | ||
| common.c | ||
| Makefile.objs | ||
| nbd-internal.h | ||
| server.c | ||
| trace-events | ||