iscsi : default iscsi mode page settings.

Binford, Charles CBinford at pirus.com
Thu Sep 20 07:32:35 PDT 2001


* From the T10 Reflector (t10 at t10.org), posted by:
* "Binford, Charles" <CBinford at Pirus.com>
*
I agree that iSCSI should not be calling out defaults for mode page field
values.  For example 3.1.1 says "The default value assumed for a new iSCSI
session (I_T nexus) is 512 units."  

This is counter to the way SCSI mode pages work.  If a SCSI initiator issues
a mode select to a mode page with the SAVE option enable, the "value assumed
for a new iSCSI session ..." (with the initiator) had better be the saved
value.  A transport such as iSCSI can't redefine how the upper layer
operates.  SCSI mode pages have a well defined behavior involving Current,
Saved, and Default pages.

I would prefer iSCSI to be silent on the "default value" of any mode page
field.  In SCSI (regardless of the transport) the default value of any given
mode page field is up to the target vendor.  Initiators choose to either a)
accept the default, b) temporarily change the current setting by issuing a
mode select (without save), or c) change the current setting via mode select
with the Save option, requesting the target to remember the new value after
any resets, power cycles, or in the case of iSCSI, new "sessions".

 
Also, on a related note, version 7-93

Appendix D. Login/Text Operational Keys
22 ImmediateData

Contains the folling text:
"This field sets the D field in the Disconnect-Reconnect mode page. The
value one of the D field means ImmediateData=no."

First, there is no 'D' field.  

Second, this appears to be the last text parameter that tries to modify a
mode page.  I agree with the move to separate text parameters from mode
pages and feel that there is no need for this one to be associated with any
mode page.



Charles Binford
Pirus Networks
316.315.0382 x222


-----Original Message-----
From: Santosh Rao [mailto:santoshr at cup.hp.com]
Sent: Wednesday, September 19, 2001 7:04 PM
To: IPS Reflector
Subject: iscsi : default iscsi mode page settings.


All,

Speaking on the subject of default settings, some questions on recent
changes to the iscsi mode select pages......

1) Section 3 of rev 7.94 of the iscsi draft attempts to call out default
values for the iscsi  mode pages. Per my understanding, there are no
defaults for SCSI mode pages, and all the setting are assumed to be
disabled, unless explicitly turned on/enabled through a mode select.

IOW, the keys in scsi mode pages are defined to be enabling certain
features and the default settings are that these features are turned off
unless a mode select is explicitly used to enable them.

However, the iscsi mode pages seems to be using the opposite policy and
is advertising default settings for mode pages, that too, agreesive ones
at that! IOW, an initiator implemenation has to explicitly issue a mode
select to disable/turn_off features rather than issue a mode select to
turn them on.

Here's a few examples :

   * default MaximumBurstSize : 512 units
   * default EMDP : 1 (i.e. modify data pointers is enabled by default
!)
   * default FirstBurstSize : 128 units. (i.e. initiators MUST use
solicited
     data, unless they explicitly turn it off using mode select, since,
not
     sending solicited data when it has been negotiated implies a target
may
     abort the I/O.

I suggest that in keeping with the scsi mode pages, NO default settings
be advertised for any iscsi mode
pages. i.e. all defaults are conservative (set to 0), unless explicitly
turned on thru a mode select.

Comments ?

Regards,
Santosh

"Mallikarjun C." wrote:

> Matthew,
>
> I completely agree that the default should be "no"!  I pointed this
> out sometime ago myself.  Apart from what you point out, the default
> setting for "ImmediateData" seems to be at variance with the
> conservative default for "InitialR2T" ("yes").
>
> Julian, could you please consider this request?
>
> Regards.
> --
> Mallikarjun

> "BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)" wrote:
> >
> > Julian,
> >
> > Appendix D24 (ImmediateData) does not describe the result of
negotiation if
> > the two sides differ. I presume that since the default is "yes",
then only
> > if both sides agree to "no" is immediate data turned off.  Please
can this
> > be stated.
> >
> > Additionally, I feel that the default value for ImmediateData should
be
> > "no".
> >
> > Similarly, there is no statement on MaxOutstandingR2T.  Presumable
the
> > minimum is selected.

*
* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo at t10.org




More information about the T10 mailing list