RE: iSCSI: Use of the A bit

["John Hufferd" <hufferd@us.ibm.com>]

To any specific user, performance is judged by their own performance not
the performance of the link or the storage controller over all.

When you read an at-distance disk, and its performance sucks, because of a
Send and Ack design, you will not have good things to say about iSCSI.  And
it is not easy to tell what the problem is.  And it is not really possible
for a customer to determine before purchase that he is buying a poorly
designed target HBA or chip.  In fact they may have tested it out in a
local environment and found it OK, and then when contacted by a remote
initiator, that remote unit is very disappointed.  This gives everyone a
bad name.  Even if the Links are well used and the overall efficiency of
the storage controller good, if the individual performance is bad, iSCSI
will be seen to be bad.

Send and Ack designs are bad designs, and in almost all cases are not
needed.  And it gives iSCSI a bad name in the environment we have been
claiming as our own, the at-distance environment.

.
.
.
John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/SSG San Jose Ca
Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
Home Office (408) 997-6136, Cell: (408) 499-9702
Internet address: hufferd@us.ibm.com


Martins Krikis <mkrikis@yahoo.com>@ece.cmu.edu on 03/15/2002 01:45:14 PM

Sent by:    owner-ips@ece.cmu.edu


To:    ips@ece.cmu.edu
cc:
Subject:    RE: iSCSI: Use of the A bit



Just a quick note. Some people have implied that the
use of A-bit necessarily means poor performance.

I would like to disagree. If the target is just
sitting there waiting for acknowledgment to come
back before sending the next Data-In sequence, then
yes, that's poor performance. But I can imagine
targets that keep sending data while waiting for
the outstanding acknowledgments. Yes, it requires
more buffer space, but the acknowledgments are
actually helpful in managing that bigger buffer space.
It's a very similar situation to multiple outstanding
R2T-s. Hopefully everybody agrees that there may be
benefits to those.

Or am I just missing something again?

  Martins Krikis, Intel Corp.

Disclaimer: These opinions are mine and may not
            reflect those of my employer.




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