iSCSI : login keys & mode page settings

Santosh Rao santoshr at cup.hp.com
Tue Apr 17 17:19:27 PDT 2001


* From the T10 Reflector (t10 at t10.org), posted by:
* Santosh Rao <santoshr at cup.hp.com>
*
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All/Julian,

The iSCSI draft is lacking sufficient description on the subject of mode
page settings specific to iSCSI, their corresponding iSCSI login keys
and the interactions between these 2 mechanisms. Specific comments
enclosed below in that regard :


1) The iSCSI draft needs to describe the layout of the protocol specific
mode pages, namely, disconnect-reconnect mode page, protocol specific
lun page and protocol specific port page as applicable to iSCSI. Such a
figurative and textual description should be along the lines of that in
FCP-2 Section 10.


2) Specifically, the iSCSI draft lacks the description of the layout of
the protocol specific lun page and in its absence, then describes a
field from this page called
EnableCmdRn. This field is non-existent in the SPC-2 description of this
page in Section 8.3.10.


3) On a side note, the EnableCmdRN  & CmdRN fields should be re-named to
EnableCRN and CRN to reflect the same semantics and context as the CRN
defined in SAM-2 and FCP-2.


4) The EnableCmdRN login key should be removed from the list of iSCSI
login keys as this is a per-LUN key and iSCSI login keys have the scope
of a session. IOW, EnableCmdRN should be negotiated through a mode
select only and not through iSCSI login.


5) On a more fundamental note, should iSCSI allow for 2 levels of
control of I-T[-L] nexus operational parameters thru both the mode
select/sense scsi mechanisms and iSCSI login key mechanisms ?

Ex :
---------------------------------------------------------------------
iSCSI login key         SCSI mode page parameter
---------------------------------------------------------------------
DataPDULength           - Max Burst Size (Disc-reconn mode page)
FirstBurstSize          - First Burst Size (disc-reconn mode page)
InDataOrder             - EMDP (disconn-reconn mode page)
EnableCmdRN             - Enable CRN (LUN control mode page)

If such control is to be allowed at both the SCSI ULP and iSCSI
transport layers, a communication mechanism should be defined to
synchronize the state of these operational parameters across the 2
layers when a change is made in either layer through its corresponding
mechanisms.

Ex : 
a) Change thru iSCSI login key should result in an up call to update the
SCSI ULP.
b) Change made thru mode select should result in a down call to update
iSCSI LLP.
c) Change thru iSCSI login key should result in an up call to SCSI ULP
to cause a UNIT ATTENTION indicating "Mode Parameters Changed".


6) If such a level of dual control is provided, the iSCSI login
keys listed above be made LO (leading only) to allow for changes to
operational parameters only during session login. This is to
minimize/eliminate disruption of ongoing I/O activity that occurs due to
the generation of a UNIT ATTENTION CHECK CONDITION when any change is
made to the above paramters.


7) If only 1 mechanism of control is desired, which of the following
alternatives is desirable :
i) Only settable thru mode select and seen thru mode sense 
Pros :
------
- Allows 1 mechanism of control.
- Removes the need for synchronization of these values across SCSI ULP &
iSCSI.

Cons :
------
- Requires setting for all LUNs to enable for the entire session.


ii) Settable thru iSCSI and also settable/viewable thru mode sense.
Pros :
------
- flexible and allows control thru both scsi & iSCSI.
Cons :
------
- Can lead to synchronization overheads. 
- Needs SP(save page) setting also to be communicated in synching iSCSI
login to mode page values.


iii) Only settable thru iSCSI and viewable thru mode sense.
Pros:
-----
- Single mechanism of modification avoiding synchronization issues in
setting.
Cons :
------
- Denies traditional mechanism of modification. (mode sense).
- May break existing applns if enforced thru SPC-2.
- Requires changes to SPC-2.
- iSCSI compliance requires changes in SCSI ULP for SPC-2 compliance of
the change to not use mode select for parameter changes that are shared
with iSCSI.


8) If these operational parameters are allowed to be set thru iSCSI
login and they also impact mode page settings, iSCSI spec should
describe the scope of the mode page setting in terms of whether this
setting is a saved page setting or not ?

9) Should saved page settings be allowed thru iSCSI ?


- Santosh


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