Re: iSCSI - Change Proposal X bit

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

Julian Satran wrote:
> 
> However in order to drop "old" commands that might in the pipe on a
> sluggish connection - removing the X bit will require the initiator to
> issue an immediate NOP requiring a NOP response on every open connection
> whenever CmdSN wraps around (becomes equal to InitCmdSN).

Julian,

Can you please explain further the corner case you are describing above
? Are you suggesting that special action should be taken every time
CmdSN wraps around, in case there were holes in the CmdSN sequence at
the wrap time ? Why is that ?

Here's my understanding of how this plays out :

Rule 1) 
The CmdSN management rules at the target should be handling CmdSN wrap
case and the initiator cannot issue more than 2^32 -1 commands beyond
the last ExpCmdSN update it has received from the target, since the
target MUST NOT transmit a MaxCmdSN that is more than 2**31 - 1 above
the last ExpCmdSN. (per Section 2.2.2.1)

Rule 2)
Any holes that occur in the CmdSN sequence are attempted to be plugged
by the initiator by re-issuing the original command. If the CmdSN never
got acknowledged and the I/O's ULP timeout expired, the initiator MUST
perform session recovery. (per Section 8.6)


Thus, going by the above 2 rules, if the CmdSN sequence wraps upto
ExpCmdSN, the initiator will not be able to issue further commands,
since the target will keep the CmdSN window closed. The window can only
re-open when the CmdSN holes are plugged allowing ExpCmdSN and thereby,
MaxCmdSN to advance.  (rule 1 above).

Under the above circumstances, the initiator will possibly try to plug
the CmdSN hole by re-issuing the original command. It may do this 1 or
more times before its ULP timeout expires. Either the holes get plugged
and the windoe re-opens, or ULP timeout occurs without the corresponding
CmdSN for that I/O having been acknowledged, resulting in session
logout. (rule 2 above).

What is required over and beyond the above ? Why does removal of X-bit
require an immediate NOP to be issued every time CmdSN wraps and a hole
exists in the CmdSN sequence (??). 

Regards,
Santosh

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