FCP-2 problem

Matt Wakeley matt_wakeley at agilent.com
Wed Jun 14 20:39:22 PDT 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* Matt Wakeley <matt_wakeley at agilent.com>
*
> >Does use of FCP_CONF_REQ on the response resolve the issue? The target will
> >release the resource after receiving FCP_CONF and the OXID is free. The REC
> >sent after the lost command will then not receive an ACC, as the OXID is
> >invalid.


> I had kicked this idea around too. This makes it better, but not perfect. The
> issue I came up with is:
>
> If the FCP_CONF and the second command are dropped, you have the same problem.
> The REC will end up recovering the first command, while the initiator thinks the
> second command has been completed.

One way to do this is, for commands that do not involve data transfer, require that
the target request FCP_CONF, and require the initiator perform an REC after sending
the FCP_CONF, to verify that the FCP_CONF got there before reusing the OX_ID.  Ugly,
but I think it'll work if we don't want to change the REC or SRR payloads.

McGRATH:
>  So another solution is to make sure you
>  do not reuse an OX_ID until you are really sure that this sort of problem
>  cannot occur.

And how do you determine that?

> In this case, using two values in alteration would work OK.

I don't think so.  Consider the following:

Initiator sends SPACE command (OX_ID=1).

SPACE command completes, and target sends response (and keeps status of the exchange
around).
<just for completeness, let's say the FCP_RSP requested a FCP_CONF, which the
initiator sent, but was lost, so the target is retaining the status of OX_ID=1>

Initiator sends WRITE command (OX_ID=2) (target is performing command queuing, so
the status of exchage with OX_ID=1 is not deallocated)

WRITE command completes, and target sends response (and keeps status of the exchange
around).

Initiator sends SPACE command (OX_ID=1), but the command is lost... same issue as
before. The target still retains the state of the original space command.

-Matt Wakeley

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