Re: iSCSI: I/O (command) recovery

[Matt Wakeley <matt_wakeley@agilent.com>]

Santosh Rao wrote:
> 
> Matt,
> 
> This is a good write-up and clearly outlines the behaviour. Just a couple of
> comments below.
> 
> Regards,
> Santosh
> 
> Matt Wakeley wrote:
> 
> > 4- The target iSCSI sends the read data and status to the initiator, but does
> > *not* deallocate the resources associated with the I/O.
> 
> The draft is ambiguous about (4). It states that the numbering MUST be supported,
> but the data and status recovery MAY be supported. This means initiators
> & targets MUST use StatSN/ExpStatSN. However, the target is allowed to
> de-allocate the resources associated with the I/O without waiting for ExpStatSN
> ack.

That's correct.  There is nothing in the document that states that retry must
be supported.  There should probably be a negotiation key that a target uses
to indicate if it supports retry or not.

> > If some error occurs such that the initiator iSCSI does not receive all the
> > data and status, then the initiator iSCSI performs whatever
> > link/connection/session recovery to re-establish communication with the
> > target.  At that point, the initiator iSCSI layer re-issues the I/O with the
> > retry bit set.
> >
> > - At this point, the target iSCSI layer (*not* the SCSI layer) recognizes the
> > command is a duplicate (because the retry bit is set), and instead of sending
> 
> Again, the wording for the above in the draft could do with some clarity (Section
> 1.2.2.1). It is not clear from the draft whether CmdSNs that are outside the
> (ExpCmdSN, MaxCmdSN) window are to be accepted if the retry bit is set.

Well, in my mind, the retried command will either have a CmdSN of zero, or a
(new/different) valid value within the current range (the next available
CmdSN).  I don't think there should ever be duplicates.

> My interpretation of :
>  "- 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)."
> 
> is :
> - ignore all commands outside the (ExpCmdSN, MaxCmdSN) range.
> - ignore all duplicate commands within the range that are not flagged with the
>    retry
> 
> So, if a command with the "retry" bit is set is received outside the (ExpCmdSN,
> MaxCmdSN) window, this causes the target to drop the received retry. The wording
> on this could do with further clarity.
> 
> >
> > For a Target Write operation:
> >
> > 8- The target iSCSI sends the status to the initiator (and saves it in the
> > iSCSI allocated resources for the I/O), but does *not* deallocate the
> > resources associated with the I/O.
> 
> Same comment as given for (4) under read operation.

Same reply as for (4) under read....

-Matt