RE: Command Queue Depth (was asymmetric/Symmetric)

[julian_satran@il.ibm.com]

Peter,

I fact we did. The SBP solution is for the target to grab the commands from
initiator memory.
Obviously that doesn't work too good if you attempt to "hide latency" - so
we attempted
a scheme with command credits - to allow several commands to be in flight.
But then we observed that with one control connection (that was before
Adelaide)
the TCP receiver window will give us this function for free.
And as things are now we still consider that the TCP receive window (even
for several
connections) gives you the same overal effect (through back pressure).
The trouble we are having is with the blocking conditions out of order
delivery may cause
and how to best avoid them.  We have now decent mechanisms to avoid/handle
deadlocks
for both the asymmetric and the symmetric scheme - although they are
definitely "cleaner"
for the asymmetric scheme (you do things in the order mandated by the
control channel).

Julo

Peter Johansson <PJohansson@ACM.org> on 10/09/2000 14:38:07

Please respond to Peter Johansson <PJohansson@ACM.org>

To:   IP Storage <IPS@ece.cmu.edu>
cc:    (bcc: Julian Satran/Haifa/IBM)
Subject:  RE: Command Queue Depth   (was asymmetric/Symmetric)




At 08:06 PM 9/6/00, Jim McGrath wrote:

>The issue of buffer space allocation for multiple initiators has a long
>and troubled history in SCSI.  We have never been able to come up with a
>good answer.
>
>Note: the same problem has plagued other attempts to allocate device
>resources between multiple initiators, like command queue space.  In
>general policies with respect to multiple initiators are not really
>standard in the SCSI world.

Jim's response is correct as far as it goes, but it ignores the solution
adopted in SBP-2, which neatly finesses these issues by leaving the
resources in the initiator.

In fairness, it's likely that the appropriateness (or lack thereof) of an
iSCSI solution similar to SBP-2 has been ignored in these discussions
because SBP-2 has been less widely implemented than parallel SCSI or FCP.

Is it possible that the SBP-2 approach is relevant to iSCSI? That is, put
the initiator in the role of "memory server" and let the target manage when
and how its own resources are used to effect the transfer of data. This
might make sense if the model places more of the assumed resources
(principally memory) in the initiator.

Can this be effected efficiently?




Regards,

Peter Johansson

Congruent Software, Inc.
98 Colorado Avenue
Berkeley, CA  94707

(510) 527-3926
(510) 527-3856 FAX

PJohansson@ACM.org