RE: iSCSI:flow control, acknowledgement, and a deterministic reco very

[Charles Monia <cmonia@NishanSystems.com>]

> TASK_SET_FULL *is* a flow control mechanism, albeit a
> rather primitive one - the Initiator issues commands until
> it gets a FULL response and hence knows the capacity
> of the Target's queue.

Using task set full to probe in this way is a guess at best. It's certainly
not behavior specified by any standard. Getting a task full after "N"
commands does not guarantee that the LU will have room for "N" commands in
the future. For efficiency reasons, logical units are loath to make any such
resource guarantee to individual initiators.  Earlier postings have
discussed this at some length.

> Santosh did not propose to remove CmdSN, although he
> did point out an issue that needs attention - we need
> to say something about the interaction of CmdSN with
> TASK_SET_FULL since both mechanisms may be implemented.
> In particular, it's possible to use CmdSN for delivery
> ACKs and TASK_SET_FULL for flow control, although that's
> a "peculiar" combination (and perhaps a SHOULD NOT?).
> 

The only thing to be said is that there is no interaction.  For example, an
iSCSI gateway fronting a bunch of FC devices has no real way to tell how
many commands each device has room for.  So, a logical unit may return a
TASK_SET_FULL response even though the CmdSN window was wide open.
Naturally, the converse is also true. I.e., the CmdSN window may close even
though logical units have room for additional commands.

Charles