Re: iSCSI Requirements Draft - Informal WG Last Call

[Sandeep Joshi <sandeepj@research.bell-labs.com>]

Julian,

I believe you mean "pipelined delivery" as opposed to "ordered
delivery".  The latter is more strict and will introduce pipeline
stalls which we would wish to avoid.  The flow control mechanism
is getting tied into introducing false sequentiality.

As an aside, a colleague here wanted to know if we had considered
having per-LUN ordering as opposed to per-session command ordering.
The pipeline could also slow down because of an aging disk or a 
heavily loaded disk.  I do see some discussions at the Haifa meeting
  http://www.ece.cmu.edu/~ips/meetingMinutes/06.20.2000.txt

Was it just added complexity which forced the per-session choice?

> Point 3.4.3 brings up ordering of commands per LUN or ordering of commands
> per session?
> MW - reorder requirements - second before the first one
> RH - does anybody disagree with 3.4.3?
> CS - No. It basically says that you support SCSI queuing
> JH - In the wide area, a method of pipelining commands and responses
> is required
> CS - the requirement is more complex than saying you just support
> SCSI queing.
> RH - delivering commands in order never hurts
> MT - Why keep order between logical units in commands?
> RH - SAM-2 does not require order between LUNs. However, it may make
> target implemetnation easier.

thanks,
-Sandeep


julian_satran@il.ibm.com wrote:
> 
> 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