Re: Data in SCSI Response or SCSI Data

[julian_satran@il.ibm.com]

Steph,

I assume that your hardware knows when it is going to send the last SCSI
datagram.
In this case it can insert the status in the datagram header (not at the
end) -
(and BTW that is how the current draft assumes that things will be done).

If you have to send sense-data (bad status) then you will send a separate
datagram.

Julo

Stephen Bailey <steph@cs.uchicago.edu> on 25/08/2000 22:12:09

Please respond to Stephen Bailey <steph@cs.uchicago.edu>

To:   ips@ece.cmu.edu
cc:    (bcc: Julian Satran/Haifa/IBM)
Subject:  Re: Data in SCSI Response or SCSI Data




> Is there anything preventing your hypothetical hardware implementor
> to send always good status within the last block of data?

It depends upon the RDMA mechanism you are using.  I admit that I have
not studied them in detail, other than ST of course, which is
essentially an transport protocol based upon a particular RDMA
mechanism.  I will try to do so soon to determine if I'm all wet.

However, assuming that the RDMA mechanism operates on a per-datagram
basis, I can only imagine that you will not be able to append
`general delivery' data to the end of an RDMA datagram.  In this case,
you will need a separate datagram to ensure that status is delivered
through a separate path from the data.

So, while I think concatenating status and data in its general form is
not a good idea, a good-status fast path (like the success bit) is
definitely the right way to think about it.  Nonsuccess SCSI status is
so rare that any compromise you can make in the nonsuccess path to
make the success path go faster is worth it.

Steph