Re: iSCSI Requirements Draft - Informal WG Last Call

[julian_satran@il.ibm.com]

Ordered delivery of commands to ANY TYPE of devices will increase in
importance as network speeds increase and the need to hide latency
increases.

Today databases don't use queuing and rely and trickle the commands to
devices 1 by 1 to ensure atomicity and order.
As latency will become the determining factor in performance this is bound
to change.

SCSI has done an excellent job in defining the queueing mechanism. We have
to make it work with good performance in our environment.



Julo

Santosh Rao <santoshr@cup.hp.com> on 13/04/2001 04:33:45

Please respond to Santosh Rao <santoshr@cup.hp.com>

To:   ips@ece.cmu.edu
cc:   Black_David@emc.com
Subject:  Re: iSCSI Requirements Draft - Informal WG Last Call




David & All,

I object to the following requirement :

" MUST support ordered delivery of SCSI commands from the initiator to
the
  target, to support SCSI Task Queuing. "

Ordered delivery is not a requirement for disk based applications and
non tagged queueing tape applications, which form the majority of
today's data traffic.

To impose strict ordering (even in the presence of errors ?) as a MUST
is penalizing the majority of today's data traffic that does not expect
ordering from the SCSI subsystem.

I am particularly concerned about the effect of the above requirement in
the presence of errors. Does iSCSI expect strict ordering to be
maintained even when individual I/O errors like ULP timeout occur ?

On a ULP timeout (caused by, say, a hole in CmdSN), the initiator may
choose not to retry the command, but instead, error it back to the ULP.
In such a case, it can plug the hole in CmdSN with a NOP-OUT.

The above requirement is not feasible to be met under such circumstances
and others similar to this. Mandating strict ordering on ULP timeouts
implies a session level error recovery on any individual I/O being
failed back from iSCSI to SCSI ULP. This is a very heavy hammer to use
as error recovery and should not be imposed.

The above requirement must be changed to :
" SHOULD support ordered delivery of SCSI commands from the initiator to
the
  target, to support SCSI Task Queuing. "

- Santosh



Black_David@emc.com wrote:
>
> It is intended to submit draft-ietf-ips-iscsi-reqmts-02.txt
> as an Informational RFC. There is no formal requirement for
> a WG Last Call, but if you have any further substantive comments
> on the document please raise them on this list within the next
> two weeks, i.e. by April 27th at the latest.
>
> If you have typographical/editorial comments please send them
> direct to the document's author, Marjorie Krueger
> <marjorie_krueger@hp.com>.
>
> Thanks,
> --David and Elizabeth, IPS WG co-chairs
 - santoshr.vcf