Re: iSCSI : default iscsi mode page settings.

[Santosh Rao <santoshr@cup.hp.com>]

Julian Satran wrote:

> Santosh,
>
> Please help this list by pointing to the specific SCSI standards where the
> mode page settings a) forbid default values

Julian,

I can't find anything in the scsi protocol standards (SPC-2, SAM-2)  that explicitly
forbid default setting specification for mode pages. However, per my understanding, this
is not the usual practice and mode page defaults are device specific. [which is also
what Charles Binford & Bob Snively seemed to imply in their mails.)

However, even if iscsi did recommend a profile of mode page defaults in the draft, such
defaults are not to be binding on implementations and should only be suggested as a
recommendation. i.e. the draft must not require that all devices use the same values in
their default mode pages.

Further, such defaults need to allow initiators to function correctly without requiring
explicit mode sense/select. Thus, any such default must be conservative in its settings
for EMDP and FirstBurstSize. (both set to 0 by default).


> b) specify what the initial
> (power-on, connection establishment) values have  to be for any protocol.

There's a reference to this in Section 7.8.6 of SPC-2 Rev 20 (dated 18 July, 2001).

Thanks,
Santosh


>
>      Regards,
>      Julo
>
>
>                     Santosh Rao
>                     <santoshr@cup.       To:     IPS Reflector <ips@ece.cmu.edu>
>                     hp.com>              cc:
>                     Sent by:             Subject:     iscsi : default iscsi mode page
>                     owner-ips@ece.        settings.
>                     cmu.edu
>
>
>                     20-09-01 03:04
>                     Please respond
>                     to Santosh Rao
>
>
>
> 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.
>
>  - santoshr.vcf
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