RE: iSCSI:flow control, acknowledgement, and a deterministic recovery

[Black_David@emc.com]

In lieu of the requested translation ...

> If you do not reject commands pending within the sequencer, then you will
> need to log these out of sequence events to ensure the command taken out
of
> sequence will not cause a hole later. 

Doug is assuming that the sequencer that issues commands runs off of CmdSN.
Needless to say, that's not necessary, as the sequencer could run off
whatever
it likes (e.g., pointers in memory).  If the sequencer runs off something
internal
instead of CmdSN, immediate commands don't cause "holes" in its sequence
because the CmdSN-based "sequence" compresses them out as commands
are released to the sequencer from the code that handles CmdSN-related
state.

As near as I can tell, the entire discussion of rejecting queued commands
when an immediate command shows up is based
on this implementation-specific assumption about not only the existence of
a "sequencer" but also the use of CmdSN to implement it.  While those are
valid ways to design a system, neither are necessary.  CmdSN tracking is
only necessary in the portion of the system that generates the ExpCmdSN
and MaxCmdSN responses and holds commands that have to wait for missing
ones to show up.  An array-based implementation rather than a queue-based
sequencer can handle this without the command rejection side-effects of
Doug's envisioned sequencer, and there are doubtless more clever ways to do
it.

Unless someone other than Doug wants to speak up on the importance
of a CmdSN-based sequencer implementation, I think discussion of that
needs to be dropped, and changes beyond the use of an immediate
flag instead of a zero CmdSN set aside as only being needed by this
specific implementation and not of general applicability.  This includes
Doug's latest message about commands being "rejected for replay at a
new sequence".

Thanks,
--David

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David L. Black, Senior Technologist
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