Hi Mike.
On Tue, Dec 04, 2007 at 10:25:29AM -0500, Mike Snitzer (snitzer@gmail.com) wrote:
In this case sync bit would only imply waiting until all pending
requests reached remote nodes. This is not implemented yet.
Order of the requests for given node is guaranteed by DST core,
it is possible to perform multiple requests in parallel for/from
different nodes.
In the more generic case it should wait until data has reached media,
i.e. perform flushing.
I did not implement that since actually no multiple-device system in
Linux supports barriers (please note, that in this discussion sync bit
actually means a barrier in the block layer).
Protocol changes are pretty trivial and are absolutely transparent for
the DST core - only remote targets (both userspace and kernelspace)
should be changed to invoke ->issue_flush_fn() callback when needed for
underlying device and do not process new requests until flush completed.
Thus barrier bit can be attached to data packets and can also be single
requests without data.
DST will continue to collect data, but will not send it to remote nodes
(actually it can send it, but data will not be processed and will stay
in the remote's receiving queue). This is a main concern about barrier -
should or not main node continue to process requests if previous ones
have not reached media yet, thus I have not yet implemented barriers.
--
Evgeniy Polyakov
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