Third party persistent reservations source I_T nexus function of XCopy

Kevin D Butt kdbutt at us.ibm.com
Tue Apr 30 09:19:27 PDT 2013


Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1304300_f.htm">HTML-formatted message</a>

Thanks Ralph!
I have more questions for all.
My engineer came back to me claiming that there are issues with the entire 
reservation model of extended copy.
The Register function described in 6.4.6.17 Register persistent 
reservation key function (DESCRIPTOR TYPE CODE 14h) is available for the 
application client to tell the copy manager to register with the CSCD but 
there is no function to tell the copy manager to reserve the CSCD. 
Therefore, the only use for the existing register function (14h) is to 
join an existing All Registrants or Registrants Only reservation that 
already exists on the CSCD.  Furthermore, there is no explicitly stated 
method for leaving that reservation or unregistering.  One might be 
tempted to send an additional register function (14h) with a zero key as 
the last segment descriptor but that would not work should a failure occur 
in a prior segment.  Then you would be back to the original issue of 
having a registration hanging around with no way to remove it (without 
PREEMPT or CLEAR).
The other reservation function is 6.4.6.18 Third party persistent 
reservations source I_T nexus function (DESCRIPTOR TYPE CODE 15h) which is 
only valid for the exclusive access type reservations.	Part of the 
description of this function restricts it to tape devices and does not 
allow disk devices.  This seems to be overly restrictive.  The other issue 
is how the copy manager gets this reservation in the first place.  The 
only way would seem to have the application client perform a REGISTER AND 
MOVE on the CSCD to the copy manager.  However, this requires the 
application client to know the relative target port of the CSCD and the 
initiator port that the copy manager will be using.  The application 
client does not know this information and so there seems to be no way for 
the copy manager to ever have a reservation with the CSCD.
Are we missing something, or is the reservation model for extended copy 
truly incomplete?
It seems that at a minimum there should be a new function to reserve the 
CSCD and a way to release the CSCD once the copy manager is done with it. 
The release could be a new bit in the existing register function that 
indicates that the copy manager shall attempt to release the CSCD after 
all segments have been processed - similar to the instructions to do the 
Third party persistent reservations source I_T nexus function (15h) after 
all segments have been processed.
Thanks,
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
Data Protection & Retention
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Tel: 520-799-5280
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at us.ibm.com
http://www-03.ibm.com/servers/storage/ 
From:	Ralph Weber <roweber at ieee.org>
To:	T10 Reflector <t10 at t10.org>, 
Date:	04/29/2013 07:08 PM
Subject:	Re: Third party persistent reservations source I_T nexus 
function of XCopy
Sent by:	owner-t10 at t10.org
* From the T10 Reflector (t10 at t10.org), posted by:
* Ralph Weber <roweber at ieee.org>
*
I believe the "intent" mentioned in question 1 matches the rather 
lengthy description that follows the question (up to, but not including, 
question 2).
I believe that any errors encountered during the processing of a Third 
party persistent reservations source I_T nexus segment descriptor should 
not overwrite an error that has already been encountered prior to the 
processing of the descriptor. There are other ways to determine the 
state of the persistent reservation, but there are few, if any, ways to 
determine the nature of the first error.
This makes the answers to questions 2 and 3 be the following:
Question 2: The COMMAND-SPECIFIC INFORMATION field shall not be altered.
Question 3: The COMMAND-SPECIFIC INFORMATION field shall set to the 
relative position of the first segment descriptor that detected an error.
All the best,
.Ralph
On 4/26/2013 5:43 PM, Kevin D Butt wrote:
> One of my engineers asked me about a function in extended copy that is 
> confusing him.  It confuses me too and I would like to know what the 
> expected behavior is.
>
> SPC-4 states:
> <<
> 6.4.6.18 Third party persistent reservations source I_T nexus function
>
> The segment descriptor format shown in table 165 instructs the copy 
> manager to send a PERSISTENT RESERVATION OUT command with REGISTER AND 
> MOVE service action (see 5.13.8) with the specified I_T nexus after 
> all other segment descriptors have been processed. If an error is 
> detected any time after receiving a third party persistent source 
> reservation I_T nexus segment descriptor, the PERSISTENT RESERVATION 
> OUT command REGISTER AND MOVE service action shall be processed before 
> the copy operation (see 5.17.4.3) originated by the EXTENDED COPY 
> command is completed.
>
> This segment descriptor should be placed at or near the beginning of 
> the list of segment descriptors to assure the copy manager processes 
> the PERSISTENT RESERVATION OUT command with REGISTER AND MOVE service 
> action in the event of an error that terminates the processing of 
> segment descriptors. If an error is detected in a segment descriptor 
> and third party persistent reservations source I_T nexus segment 
> descriptor has not been processed, the copy manager shall not send a 
> PERSISTENT RESERVATION OUT command with REGISTER AND MOVE service 
action.
>
> Placing more than one source third party persistent reservations 
> source I_T nexus segment descriptor in the list of descriptors is not 
> an error. All source third party persistent reservations source I_T 
> nexus segment descriptors known to the copy manager shall be processed 
> after all other segment descriptors have been processed.
> >>
>
> This appears to say that this function requires a REGISTER AND MOVE to 
> be process after all other segments have been processed, presumably to 
> transfer the reservation after all the copy functions have been 
> processed.  This segment should be early in the list so it can be 
> processed to transfer the reservation even if there is a failure while 
> attempting to process any of the segments that occur in the list after 
> this function.  And, there is a requirement to process this segment 
> even if there is a failure in one of the other segments.  However, if 
> the failure occurs in one of the segments prior to this function, then 
> the REGISTER AND MOVE shall not be processed.  Further, there are 
> allowed to be multiple instances of this function and all instances 
> are processed after all the other segments have been processed. 
>  Presumably, these functions would be processed in the order they 
> appear in the list.
>
> This seems to be the behavior described, but it is difficult to be 
> sure this is the intent.
> *Question 1: *Is this the intent?
>
> Assuming this is the correct behavior, then assume a parameter list 
> with 10 segments.  There is a Third party persistent reservations 
> source I_T nexus function in the third segment descriptor and another 
> that is in the sixth segment descriptor.  Further, assume that there 
> is an error in processing the eighth segment descriptor.
>
> In 5.17.7.4 EXTENDED COPY command errors detected during processing of 
> segment descriptors it states:
> After an exception condition is detected during segment descriptor 
> processing:
> b) the copy manager shall indicate the segment that was being 
> processed at the time of the exception by writing the segment number 
> to the third and fourth bytes of the COMMAND-SPECIFIC INFORMATION 
> field. The segment number is based on the relative position of the 
> segment descriptor in the parameter list (see 5.17.7.1)
> In this case the segment number put in the third and fourth bytes of 
> the COMMAND-SPECIFIC INFORMATION field is 7 (i.e., the eighth 
> descriptor).
>
> *Question 2: *What should be put into the COMMAND-SPECIFIC INFORMATION 
> field if there is also a failure in processing the REGISTER AND MOVE 
> in the third or sixth segment descriptor that is required to be 
> processed at the end after all other segments have been processed or 
> after an error occurs?
>
> *Question 3:*  What should be put into the COMMAND-SPECIFIC 
> INFORMATION field if all copy segments complete without error, but 
> then during the REGISTER AND MOVE one of the third or sixth segment 
> has an error?  If we indicate the failing Third party persistent 
> reservations source I_T nexus function then we lose the fact that all 
> the other segments completed successfully.
>
> Thanks,
>
> Kevin D. Butt
> SCSI & Fibre Channel Architect, Tape Firmware
> Data Protection & Retention
> MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
> Tel: 520-799-5280
> Fax: 520-799-2723 (T/L:321)
> Email address: kdbutt at us.ibm.com
> http://www-03.ibm.com/servers/storage/
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