Subject: RE: iscsi : iscsi parameter default values

From: "Binford, Charles" <[email-masked]>
Date: Thu, 27 Sep 2001 08:19:59 -0400

I strongly agree that EMDP should not "default" to 1.  Two reasons:

1) as I recently argued on this thread, I think it is invalid for a protocol
to define a "default" for a mode page - that's not how mode pages work.

2) **main reason** The whole concept of delivering data out order (for the
normal I/O, not error recovery) buys a lot of complexity in both initiator
and target for marginal "improvements" in buffer management and or
performance.  I'm not convinced most targets gain anything from sending or
receiving normal I/O data out of order.  Therefore I think it is a mistake
for iSCSI for force a host to be able to handle the out of order case or
else issue a mode select *every* time a new session starts up.  Keep it
SIMPLE.  I fully agree with Santosh that the defaults for login keys should
be conservative.

Charles Binford
Pirus Networks
316.315.0382 x222

-----Original Message-----
From: Santosh Rao [mailto:[email-masked]]
Sent: Wednesday, September 26, 2001 9:29 PM
To: [email-masked]
Subject: iscsi : iscsi parameter default values

All,

With the issue of mode page vs. login keys having [almost] drawn to a close,
I would like to
raise the below issues again, on the subject of default values for login
keys and other iscsi
parameters :

   * In keeping with traditional use of scsi mode pages, iscsi should not
specify any default
     settings for any mode pages that continue to exist for iscsi. "Default
values" for mode
     pages are target specific and should not be bound to the protocol
draft.

   * MORE IMPORTANTLY, I read the default for EMDP as being set to 1  :-<
This implies that
     targets must be always prepared to deal with out of order data and
initiators must be
     prepared to deal with out of order data, unless the initiator performs
a mode select to
     disable it. [which defeats all the previous advantages gained thru
mandating use of login
     keys for other negotiations.]. A default, if it were to exist, should
be 0. (in-order, by
     default).

   * Conservative specification of defaults for login keys along the
following lines :
                            MaxConnections = 1
                            FMarker = "no"
                            InitialR2T = "yes"
                            BidiInitialR2T = "yes"
                            ImmediateData = "no"
                            DataPDULength = 16
                            MaxOutstandingR2T = 1
                            DataPDUInOrder = "yes"
                            ErrorRecoveryLevel = 0
                            SessionType = "normal"

   * Should the iscsi protocol require a "Lun Control Mode Page"? IOW, is an
EnableCRN bit
     required at the transport layer ? If the device server capability is to
be negotiated , I
     suggest this bit be moved to a SCSI ULP Mode Page such as the "Control
Mode Page", through a
     T10 change as a part of the SCSI changes being driven by iscsi.

Comments ?

Thanks,
Santosh

Santosh Rao wrote:

> There are the separate issues of :
>
>    * iscsi's specification of defaults for its mode pages which is not in
line with mode page
>      usage. This impacts the target's ability to enforce values other than
the protocol
>      specified default, if the initiator were to not use mode
sense/select.
>
>    * default settings for login keys.
>
>    * Is there a need for the "LUN Control Mode Page" and whether "Enable
CRN" should be in a
>      transport specific mode page ?
>
> which need to be driven to closure as well.
>
> Regards,
> Santosh

---

From: James Smart <[email-masked]>
Date: Thu, 27 Sep 2001 09:40:38 -0400

I second the comments made by Charles. Most debates I've participated in for out
of order delivery have been more theoretical than in practice. Given the
tradeoff is significantly simpler/faster implementations for in-order delivery,
especially at the low level hardware, I agree it should be EMDP=0;

I also fully agree with the default login keys proposed by Santosh.

-- James