Re: iscsi : X bit in SCSI Command PDU.

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

Julian,

Can you please clarify in which scenarios the X bit is required in the
SCSI Command PDU ?

With the removal of command replay, a re-transmitted scsi command for
the purpose of plugging a possible hole in the target's CmdSN sequence
can be handled at the target without requiring to look at the X bit. 

I see no need to retain the X bit in the SCSI Command PDU. Further, the
X bit could also be removed from all iscsi command pdu's other than the
login pdu, wherein, the X bit has different semantics. The X bit is only
meaningful and truly required in the login command pdu (where it is used
to indicate a logout+re-login). In all other PDUs, it can be removed.

Regards,
Santosh


Julian Satran wrote:
> 
> Santosh,
> 
> I am not sure you went through all scenarios. A conversation with your
> colleague - Mallikarjun - and getting through the state table may go a
> long way to clarify the need for X.
> 
> And I am sure that by now you found yourself several .
> 
> Julo
> 
>   Santosh Rao
>   <santoshr@cup.hp.com>                   To:        IPS Reflector
>   Sent by: owner-ips@ece.cmu.edu  <ips@ece.cmu.edu>
>                                           cc:
>   06-10-01 01:56                          Subject:        iscsi : X
>   Please respond to Santosh Rao   bit in SCSI Command PDU.
> 
> 
> 
> All,
> 
> With the elimination of command relay from iscsi [in the interests of
> simplification ?], I believe that the X bit in the SCSI Command PDU
> can
> also be removed. As it exists today, the X bit is only being used for
> command restart, which is at attempt by the initiator to plug a
> potential hole in the CmdSN sequence at the target. It does this on
> failing to get an ExpCmdSN ack for a previously sent command within
> some
> timeout period.
> 
> Given the above usage of command restart, no X bit is required to be
> set
> in the SCSI Command PDU when command re-start is done.
> 
> Either :
> (a) the target had dropped the command earlier due to a digest error,
> in
> which case, the command restart plugs the CmdSN hole in the target.
> 
> [OR]
> 
> (b) the target had received the command and was working on it, when
> the
> initiator timed out too soon and attempted a command restart to plug
> [what it thought was] a possible hole in the CmdSN sequence.
> 
> In case (a), no X bit was required, since the target knows nothing of
> the original command. In case (b), no X bit is required again, since
> the
> (ExpCmdSN, MaxCmdSN) window would have advanced and the target can
> silently discard the received retry and continue working on the
> original
> command received.
> 
> Removal of the X bit in the SCSI Command PDU has the following
> benefits
> :
> 
> a) The CmdSN rules at the target are simplified. No need to look at X
> bit, only validate received CmdSN with (ExpCmdSN, MaxCmdSN) window.
> 
> b) The reject reason code "command already in progress" can be
> removed.
> There's no need for this reject reason code anymore, since X bit
> itself
> is not required, and the targets can silently discard commands outside
> the command window and continue to work on the original instance of
> the
> command already being processed at the target.
> 
> c) Less work for the target and less resources consumed since it no
> longer needs to generate a Reject PDU of type "command in progress".
> It
> can just silently discard any command PDU outside the (ExpCmdSN,
> MaxCmdSN) window.
> 
> d) Less code for the target, since it does not need :
> - any Reject code paths when it receives X bit command PDUs that are
> already in progress.
> - No special casing of CmdSN checking rules.
> - No overheads of verifying a received command based on its initiator
> task tag, to check if the task is currently active, prior to sending a
> Reject response with "command in progress".
> 
> Comments ?
> 
> Thanks,
> Santosh
> 
> --
> ##################################
> Santosh Rao
> Software Design Engineer,
> HP-UX iSCSI Driver Team,
> Hewlett Packard, Cupertino.
> email : santoshr@cup.hp.com
> Phone : 408-447-3751
> ##################################

-- 
##################################
Santosh Rao
Software Design Engineer,
HP-UX iSCSI Driver Team,
Hewlett Packard, Cupertino.
email : santoshr@cup.hp.com
Phone : 408-447-3751
##################################