Re: iSCSI : Digest Error Problems & CmdSN/ExpCmdSN window issues

[Pierre Labat <pierre_labat@hp.com>]

Santosh Rao wrote:

> Matt,
>
> (You may want to take another look at the thread.). Here's the details again :
>
> > 3) The policy of "discard and restart" is also subject some race
> > conditions in the CmdSN sliding window. At the time the digest error was
> >
> > detected at the initiator, the ExpCmdSN may not yet have acknowledged
> > that
> > command. This causes the initiator to restart the command with the same
> > Initiator Task Tag, CmdSN and "retry" bit set.

The "retry" can work and works fine only in the case it "retry" a command
that was issued on a failed connection that has been logged out.
In the other cases you are in trouble because of the ghost IOs and "retry"
MUST be avoided.

>
> >
> > However, by the time the command gets to the target, the target may have
> >
> > updated its ExpCmdSN window having sent a later PDU which updated the
> > ExpCmdSN. This results in the target discarding the received CmdSN since
> >
> > <js> at command restart you never really rely on the CmdSN. you will want
> > to check
> > the Initiator Task Tag and accept it in the above case. </js>
>
> The draft states the following on this subject :
>
> Section 5.1
> =========
> -    the initiator will reissue all outstanding commands with their
>       original Initiator Task Tag and their original CmdRN if they
>       are not acknowledged yet or a CmdRN of 0 (not-numbered) if they
>       were acknowledged; the retry (X) flag in the command PDU will
>       be set
>
> Section 1.2.2.1
> ===========
> - The target MUST silently ignore any command
>    outside this range or duplicates within the range not flagged with
>    the retry bit (the X bit in the opcode).
>
> This, to me, means :
> - ignore all commands outside the (ExpCmdSN, MaxCmdSN) range.
> - ignore all duplicate commands within the range that are not flagged with the
> retry
> bit. << "duplicate" added this time >>
>
> Can you please clarify the intent of the text ?

Duplicate means a command with the same CmdSN.

Regards,

Pierre