Re: iSCSI : Command Ordering Proposal.

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

I'd also like to point out that all this discussion on ordered task tags is
mostly of academic interest. How many implementations today use Ordered Task Tags
? I have'nt seen this in use in the implementations I've worked with.

Does anybody know of how widely used this ordered task tag feature is ? If it is
a totally un-used feature, we're adding all of this complexity to address
something that is un-used.

Regards,
Santosh


Santosh Rao wrote:

> Charles Monia wrote:
>
> >   For example, if I
> > send commands A B |C| D E, where |C| is ordered, I expect that D and E won't
> > complete before |C|.  If A B D and E arrive followed by |C|, there's no way
> > to obtain the correct behavior. Incidentally, similar constraints hold true
> > for "head of queue" tasks.
> >
>
> Charles,
>
> SAM-2 Section 7 states :
>
> "The rules for task set management apply only after the task is entered
> into the task set."
>
> This implies that the enforcement of the ordering of task tags in your above
> example is NOT based on when the initiator sent the commands, but on when
> these tasks enter the task set at the target.
>
> i.e. The initiator may send A, B, |C|, D, E, where |C| is ordered.
>
> The commands may arrive at the target in the order :
> A, E, |C|, B, D in a multi-connection session.
>
> The commands A and E are not subject to any form of ordering.
> (being simple tag commands and having no ordered tag commands
> ahead in the task set.)
>
> The commands |C|, B & D are subject to the ordering that B & D cannot
> be executed until C is first executed. Thereafter, B & D are subject to
> simple tag rules.
>
> I believe the confusion lies in whether ordered task tags imply end-to-end
> ordering or ordering within the received task set at the target. the former
> requires strict ordering on the link or re-ordering at the target, in the
> absence
> of strict link ordering. The latter does NOT require any link ordering or
> re-ordering
> at the target, since it only enforces ordering within the received task set.
>
> Comments ?
>
> Regards,
> Santosh
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