Determining application client support for CRN.

Dave Peterson dap at cisco.com
Tue Sep 25 09:31:19 PDT 2001


* From the T10 Reflector (t10 at t10.org), posted by:
* "Dave Peterson" <dap at cisco.com>
*
>> SRP does not support CRN.  It's garnered no adoption in FC and the
>> need for it is obviated by SSC-2's explicit command set for tapes
>> and by SPC-2's Extended Copy command which lets tapes serve
>> as initiators and avoid tape command ordering problems altogether.

Tape ordering problems are still an issue even with the SSC-2 explicit
command set.
For example, markers are now in the explicit write, write filemarks commands
to detect the first (and last) write type command of a write sequence has
been received.
If CRN was used the marker(s) would not be needed. Maybe its time to produce
CDBs
with embedded reference numbers.

I'm not sure how SPC-2 Extended Copy avoids tape command ordering problems
altogether.

<* Is the CRN support capability of the application client to be
<     assumed by the device server ? If not, is'nt a bit required in the
<     "Control Mode Page" or some other ULP specific mode page to test
<     for CRN support at the device server ?

<* What is the intent of the EPDC( Enable Precise Delivery Checking)
<     bit in the LUN Control Mode Page in FCP-2 ? I assume this is meant
<     to test for the CRN support at the target FCP layer [since
<     FCP implementations would treat the CRN byte as reserved in the
<     FCP_CMD IU]. I assume this is NOT meant to verify the application
<     client support, is that correct ?

CRN at the application client is optional, thus CRN support is not to be
"assumed" by the device server.
Application client determines support (or non-support) of CRN at the device
server via the EPDC bit in the logical unit control page. The application
enables precise delivery (i.e. CRN) by successfully setting the EPDC bit to
one.
The application client is then allowed to use the CRN field as specified in
FCP-2 clause 4.3

<* Is there any need for new SCSI transports to have a bit similar to
<     the EPDC bit of FCP-2, if they are guaranteed to support the CRN
<     field in their SCSI Command encapsulation IUs from their initial
<     revs ? I ask this in the context of iscsi. I also noticed that SRP
<     has chosen not to define the Protocol Specific LUN Page [which
<     would have contained something equivalent to the EPDC bit].


Dave

> -----Original Message-----
> From: owner-t10 at t10.org [mailto:owner-t10 at t10.org]On Behalf Of Elliott,
> Robert
> Sent: Monday, September 24, 2001 11:17 PM
> To: Santosh Rao; T10 Reflector
> Subject: RE: Determining application client support for CRN.
>
>
> * From the T10 Reflector (t10 at t10.org), posted by:
> * "Elliott, Robert" <Robert.Elliott at compaq.com>
> *
>
>
> > -----Original Message-----
> > From: Santosh Rao [mailto:santoshr at cup.hp.com]
> > Sent: Monday, September 24, 2001 7:57 PM
> > To: T10 Reflector
> > Subject: Determining application client support for CRN.
> >
> > I've got some questions on the CRN field as defined in SAM-2 Rev 20
> > Section 5.1 and the EPDC bit in the LUN Control Mode Page in
> > FCP-2. Your responses would be much appreciated.
> > ...
> >
> >    * Is there any need for new SCSI transports to have a bit similar
> to
> >      the EPDC bit of FCP-2, if they are guaranteed to support the CRN
> >      field in their SCSI Command encapsulation IUs from their initial
> >      revs ? I ask this in the context of iscsi. I also ced that SRP
> >      has chosen not to define the Protocol Specific LUN Page [which
> >      would have contained something equivalent to the EPDC bit].
>
> SRP does not support CRN.  It's garnered no adoption in FC and the
> need for it is obviated by SSC-2's explicit command set for tapes
> and by SPC-2's Extended Copy command which lets tapes serve
> as initiators and avoid tape command ordering problems altogether.
>
> ---
> Rob Elliott, Compaq Server Storage
> Robert.Elliott at compaq.com
>
>
> *
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