RE: A couple of iFCP questions

[Black_David@emc.com]

> It is safe to discard a translation when there are no active N_PORT
> sessions for a remote N_PORT.  In that case, the iFCP gateway SHOULD
> invalidate and remove the mapping to that remote N_PORT.

I don't believe that is always correct.  I know of Fibre Channel systems
that do something approximating a mainframe-style disconnect/reconnect
based on dropping the N_PORT session and opening up a new one if the
gap in I/O activity is sufficient.  I suspect things may be even worse in
the
tape world where the gap between sessions may be much longer (e.g.,
when the tape device is being sequentially shared among a set of
systems); is anyone on this list a tape expert who knows for certain? 
Prematurely discarding a translation will cause unexpected results in
these cases.

OTOH, there should be a solution along the lines of accumulating a
large number of "should be invalid" translations, discarding them in a
single batch and issuing the appropriate state change notifications.
Care is still required, as this "correct" operation of the (logical/virtual)
fabric may still have undesirable higher-level consequences (e.g., backup
application aborts).  I should note that this approach is unique to
Fibre Channel, as the networking analogs (e.g., running UDP through
a NAT) don't have a facility like state change notification available, and
hence generally can't recover from incorrectly discarding a translation.

> Of course,
> even if it doesn't, a device establishing a new N_PORT session has the
> responsibility of validating any pre-existing mapping by checking WWPN's
> of the remote device.

If "device" refers to the Fibre Channel device containing the N_PORT, I'm
not
sure this check is (always) done.  OTOH, if a mapping has changed, the state
change notification mechanism described in the response to the second
question will prevent this sort of problem.

The explanation of iSNS's role in change propagation seems fine, although
it makes connectivity to iSNS a requirement for operation of an iFCP
gateway -- in other words, if the iFCP gateway loses contact with iSNS,
it loses the ability to detect that its translations have become invalid.
I mostly wanted to note the contrast in availability requirements with DNS -
DNS does not have to be available after a connection has been set up.

--David
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David L. Black, Senior Technologist
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