End-to-end logical block guard checking question

Edward A. Gardner eag at ophidian.com
Fri Jan 23 09:57:37 PST 2004

* From the T10 Reflector (t10 at t10.org), posted by:
* "Edward A. Gardner" <eag at ophidian.com>
At 07:54 23-01-2004, George Penokie wrote:

>In the protection information description in SBC-2 section 4.5.1 
>Protection information overview the following statement is made:
>If the logical unit is formatted with protection information and the EMDP 
>bit is set to one in the Disconnect-Reconnect mode page (see SPC-3), then 
>checking of the logical block reference tag or the logical block guard 
>within the service delivery subsystem may cause false errors because 
>logical blocks may be transmitted out of order.

That statement has never made any sense to me.  It's no different than 
saying that if EMDP is set to one, then data may be transferred to 
incorrect data buffer offsets because logical blocks may be transmitted out 
of order.

Out of order transfers are clearly more complicated than in order 
transfers.  Any implementation that allows EMDP to be set to one must 
account for that.  It must account for it both when transferring to/from a 
data buffer and when checking protection information.

I suggest we remove that statement, as it serves no purpose.  It assumes a 
particular implementation, one that only implements in order transfers, and 
observes that it will break if presented with out of order 
transfers.  Duhhh!  If we were to correct it to be implementation neutral, 
it would amount to saying that protection checking must be implemented 
correctly or it won't work.  Not exactly a useful statement.

Incidentally, the statements that "protection information ... may be 
checked by any object along the I_T_L nexus" (first paragraph 4.5.1) and 
"checking ... within the service delivery subsystem" (paragraph George 
quoted) have some problems.  Checking protection information requires 
knowledge of the block size to determine where the protection information 
is located.  But the service delivery subsystem is unaware of the block 
size.  Only the application layer knows the block size (according the 
SAM-n), therefore only the application layer can check protection information.

Edward A. Gardner               eag at ophidian dot com
Ophidian Designs                719 593-8866 voice
1262 Hofstead Terrace           719 210-7200 cell
Colorado Springs, CO  80907

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