iSCSI:Request/Response Ordering

Edward A. Gardner eag at ophidian.com
Sun Sep 30 07:52:07 PDT 2001


* From the T10 Reflector (t10 at t10.org), posted by:
* "Edward A. Gardner" <eag at ophidian.com>
*
>Howevere i think that there should be some way in which write access to the
>target can be reserved for some initiators. ( i dont know if that already
>exists) or there should be some way of ordering of requests fos a target LU
>in task router/ task manager

I'm not certain what you're trying to get at.  There's the various forms of
reservation.  Persistent reservations have been particularly well thought
out for multi-initiator or cluster operation.

There are some inherent difficulties with asking a target to coordinate or
order writes.  From an initiator's perspective, a command is outstanding
while it is in flight to the target, while the target is queuing and
processing the command, and while status is in flight back to the initiator.
A target only has visibility to the middle part, it cannot do anything with
commands that are in flight.  For example, it does no good to require that
the target perform writes in order if they can become re-ordered while in
flight to the target.

This pretty well means that any coordination has to be done using extra
commands, not the write command itself.  The extra commands are equivalent
to a form of reserve and release.

There are some rather complex approaches that would let targets do such
things.  But it's tantamount to making targets active participants in
cluster lock managers.  Noone's wanted to pursue approaches with that level
of complexity (I think with good reason).

The accepted way to do shared access is that the hosts / initiators
coordinate everything using their lock manager.  At the target level
operations can be performed in whatever order is convenient.  A number of
vendors have built cluster software from such an approach.  It appears to
work well.

Edward A. Gardner               eag at ophidian.com
Ophidian Designs                719 593-8866 voice
1262 Hofstead Terrace           719 593-8989 fax
Colorado Springs, CO  80907     719 210-7200 cell


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