Re: iscsi : default iscsi mode page settings.

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

In addition to what Santosh said,  If I understand this right,
I think it is a problem for iSCSI to have to keep going across layers to
determine what the values are.  Since iSCSI Target will not see the CDB
that caused the values to change.

Now if the value in the mode page is only the default, that would be a
different issue.

.
.
.
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
Internet address: hufferd@us.ibm.com


Santosh Rao <santoshr@cup.hp.com>@ece.cmu.edu on 09/24/2001 12:28:43 PM

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


To:   Julian Satran/Haifa/IBM@IBMIL
cc:   ips@ece.cmu.edu
Subject:  Re: iscsi : default iscsi mode page settings.



Julian Satran wrote:

> I can sympathize with you wanting to use most of the parameters in iSCSI
-
> but the values are in fact restrictions that SCSI places on iSCSI.

Julian,

I'm confused by your response.

The SPC-2 description of Disconnect-Reconnect mode page indicates that :
"The parameters appropriate to each protocol and their interpretation for
that protocol may
be specified in the individual protocol documents".

FYI, SPI[-4] has chosen not to attach any semantics to FirstBurstSize for
the pSCSI
transport. Thus, iscsi is within its rights to declare this field as
reserved and attach no
meaning to it in the mode page. The FirstBurstSize can be negotiated during
iscsi login
through a login key.


> Nevertheless the discussion is rather academic because SCSI can hand
those
> parameters to iSCSI.

Again, I'm confused by your response. The reasons I'm suggesting the use of
a login key
instead of the mode page method are :

   * More accurate scope (applies only to this I-T nexus).

   * More optimal negotiation and reduced overhead in the establishment of
the I-T nexus. (2
     less SCSI commands per I-T nexus establishment.).

   * Enables faster I/O scan times due to lesser on-the-wire activity
during I-T nexus
     establishment.

   * Allows less room for error in the I-T nexus establishment (no
possiblity of failure to
     establish I-T nexus due to mode sense/select command failure).

   * Avoids mode select wars that can occur when target uses shared mode
pages.

   * Simpler initiator implementations since they can avoid embedding SCSI
command set
     knowledge as well as code to build/parse SCSI commands. Also, they can
avoid extra code
     that is required to snoop for CHECK CONDITION with (sense key=UA, ASC
="mode parameters
     changed") in order to re-issue a mode sense to determine new values
for FirstBurstSize.

   * Less code to interact with SCSI ULP application client to co-ordinate
the mode page
     values b/n the ULP & LLP.

   * Can use un-solicited data from the very first scsi command in the
session.

I don't consider any of the above reasons to be academic and would like to
know which ones
among the above do you believe are academic and why ?


> SCSI can handle those parameters dynamically. iSCSI may have trouble
> handling this type of negotiation dynamically over several connections.

This is exactly the kind of stuff we don't need and should actually be
trying to avoid. What
good does dynamically changing FirstBurstSize serve ? Dynamically changing
FirstBurstSize
would only be achieved with least side-effects if :
1) The mode select implementation on target is not using shared mode pages.
2) The initiator has quiesced I/O prior to issuing the mode select for the
change.

Neither of the above 2 conditions would typically apply and any dynamic
change of
FirstBurstSize would only cause initiators to see a bunch of side-effects
such as :
a) Active outbound I/Os aborted by the target with a CHECK CONDITION due to
"not enough
un-solicited data".
b) UA CHECK CONDITION for "mode parameters changed".

In the interests of simplification and avoiding disruption of active I/O,
such modifications
must be avoided as far as possible. One way to achieve that is to use a
login key and make
it LO.


>
> Resource-wise (as Bob Snively has pointed out) those are SCSI issues.
>
> A nice way out would be to ask T10 for a text mode negotiaton :-)

Once again, I'm perplexed by your response. I'm not saying that text mode
negotiation is the
reason I suggest moving this to a login key. The main objective is to
isolate such
negotiation within the iscsi layer in an iscsi specific PDU that is a part
of the iscsi
login process.

Hope you will consider all of the above factors.

Thanks,
Santosh

ps : [I wonder if there are any others on this list who care to voice their
opinion on this
issue. (??). ]