Subject: [storm] SPC-2 reserve/release: Proposed text - version 3

From: "Black, David" <[email-masked]>
Date: Tue, 25 Sep 2012 15:53:39 -0400

version 3:

Responding to Mallikarjun's comments (plus some more edits) -

> 1) IMHO, opening sentence of "I_T nexus state includes reservation state"
> is a little too broad,

I removed that sentence, removed I_T Nexus from new section title and tweaked
the persistent reservation text to talk about state in general as opposed to
I_T Nexus state in particular.

> 2) Hard reset is not an LU reset - it is either an iSCSI Target Warm Reset,
> or (more likely) an iSCSI Target Cold Reset. In either case, any of the
> three resets should clear the reserve/release state.

I removed the word "hard" - as you noted, an LU reset is sufficient to remove
a reserve/release reservation.

> 3) Should use Time2Retain+Time2Wait, not Default Time2Wait

Brilliant minds think alike - I got to that one in version 2 ;-).

> 4) Session recovery does not necessarily preserve the I_T nexus state
> (recovery may not occur within Time2Wait+Time2Retain, and may not use
> the same ISID when it occurs), but session reinstatement and session
> continuation certainly should. 

I changed "session recovery" to "session reinstatement".

> 5) Should articulate the underlying linkage between Time2Retain
> (an iSCSI concept), and the reserve/release state (a SCSI state).

I believe I also got to that one in version 2.

version 2:

I moved a paragraph break in response to Ralph's comments, and rewrote
the first additional consideration to reference the connection timeout
values in general.  I also found some minor problems that need correction
in Section 4.6.3.3 (see end of this message).

<WG chair hat off>

Here's an initial attempt to propose text to deal with the reserve/
release topic in the consolidated iSCSI draft.  Please comment,
correct, suggest edits, etc.

The new text would be a new section 4.4.3.2 that would follow 4.4.3.1:

4.4.3.1. I_T Nexus State

  Certain nexus relationships contain an explicit state (e.g.,
  initiator-specific mode pages) that may need to be preserved by
  the device server [SAM2] in a logical unit through changes or
  failures in the iSCSI layer (e.g., session failures). In order for
  that state to be restored, the iSCSI initiator should reestablish
  its session (re-login) to the same Target Portal Group using the
  previous ISID. That is, it should perform session recovery as
  described in Chapter 6. This is because the SCSI initiator port
  identifier and the SCSI target port identifier (or relative target
  port) form the datum that the SCSI logical unit device server uses
  to identify the I_T nexus.

-------------------------------

First, for clarity, the following change should be made in the
above 4.4.3.1 text to better identify the recovery mechanism:

OLD
  That is, it should perform session recovery as
  described in Chapter 6.
NEW
  That is, it should recover the session via connection
  reinstatement as described in Section 6.3.4.
END

I believe that a lower-case "should" remains appropriate for this text.

-----------------------------------

NEW TEXT (second draft):

4.4.3.2. Reservations

  There are two reservation management methods defined in the SCSI
  standards, reserve/release reservations, based on the RESERVE and
  RELEASE commands [SPC2], and persistent reservations, based on the
  PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT commands [SPC3].
  Reserve/release reservations are obsolete [SPC3] and SHOULD NOT be
  used; persistent reservations SHOULD be used instead.

  State for persistent reservations is generally required to persist
  through changes and failures at the iSCSI layer that result in
  I_T Nexus failures, see [SPC3] for details and specific requirements.

  In contrast, [SPC2] does not specify detailed persistence
  requirements for reserve/release reservation state after an I_T
  Nexus failure.  Nonetheless, when reserve/release reservations are
  supported by an iSCSI target, the preferred implementation approach
  is to preserve reserve/release reservation state along with other
  I_T Nexus state for iSCSI session reinstatement or session
  continuation via connection reinstatement.

  Two additional caveats apply to reserve/release reservations:

  - Retention of reserve/release reservation state for an extended
	period of time after connection failure may require a reset
      (e.g., LOGICAL UNIT RESET, see section 11.5) in order to remove
      that reservation state when connection reinstatement is not
      performed.  When reserve/release reservation state is retained
      after connection failure, the need for such resets may be
      reduced via suitable selection of the Time2Wait and Time2Retain
      timeout values that control connection state retention duration
      (see Section 7.5).
 
  - Reserve/release reservations may not behave as expected when
      persistent reservations are also used on the same logical unit;
      see the discussion of "Exceptions to SPC-2 RESERVE and RELEASE
      behavior" in [SPC4].

-----------------------

Reference impacts: both [SPC2] and [SPC3] need to be normative
references, but [SPC4] can be an informative reference.

-------------------------

I deliberately used "the preferred iSCSI implementation approach"
wording for reserve/release reservation state preservation requirements,
as SPC-2 is vague on this topic and I'm rather uncomfortable with placing
a requirement as strong as a "SHOULD" on an obsolete mechanism that
"SHOULD NOT" be used.

---------------

New problem in Section 4.6.3.3 - in this text:

   The Logout response indicates that the connection or session
   cleanup is completed and no other responses will arrive on the
   connection (if received on the logging out connection). In
   addition, the Logout Response indicates how long the target will
   continue to hold resources for recovery (e.g., command execution
   that continues on a new connection) in the text key Time2Retain
   and how long the initiator must wait before proceeding with
   recovery in the text key Time2Wait.

Time2Wait and Time2Retain are not text keys - they're binary fields
in the Logout Response PDU.  Two changes are in order:
	- "text key Time2Retain" -> "Time2Retain field"
	- "text key Time2Wait" -> "Time2Wait field"

Thanks,
--David
----------------------------------------------------
David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA  01748
+1 (508) 293-7953              FAX: +1 (508) 293-7786
[email-masked]        Mobile: +1 (978) 394-7754
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