LongLBA bit in READ CAPACITY (10) command

Bob Frey rtfrey at IEEE.org
Mon Sep 30 13:15:46 PDT 2002


* From the T10 Reflector (t10 at t10.org), posted by:
* Bob Frey <rtfrey at ieee.org>
*
To maintain backward compatibility with target devices that either
pre-date the LongLBA bit or don't support it and return an error at
best or a wrong result at worst, I believe it is preferable (and the
intention of the authors of the LongLBA bit) to use the READ CAPACITY
command without the LongLBA bit set first. The READ CAPACITY command
with the LongLBA bit SET should be issued only if the first READ CAPACITY
returns a Returned LBA value which indicates the field can not be
represented which is a value of all F's. (All F's may be the correct
LBA. This information will be discovered on the second READ CAPACITY
command with the LongLBA bit set.) This is my assumption from reading
SBC because I don't recall reading the proposal(s) or history that led
to the inclusion of the LoginLBA bit. My recollection is that the
SBC text states the behavior/result but does not state a preferred
order or usage of the LongLBA bit.

The direction T10 has taken to remove recommended usage/implementor's
notes that existed in SCSI-2 and earlier (which is again my recollection)
seems to me a mistake that has led to vendor interoperability issues that
could otherwise have been avoided. Vendors do read the T10 specifications
but because it takes a T10 historian/alchemist to combine T10 ingredients
together to create a working T10 potion, many vendors (and even more that
use their products) find the products fizzle.

I encourage a policy of going back to implementor's notes. I would start
with an informative or normative annex in SPC or SBC that describes LU
discovery which in my experience has been the T10 area where the majority
of interoperability issues and questions have and will continue to exist.
(As evidenced by the LUN format/HiSup and LongLBA bit questions on this
reflector.)

Here's an overview of a typical LU discovery sequence that I have never
read in a T10 document but believe I have independently arrived at after
about 8 years working on and off with initiator and target T10 implementations:

  1. INQUIRY LU 0
  2. REPORT LUNS 0 (Failing REPORT LUNS perform INQUIRY LU scanning 0-255)
  3. For each LU discovered
     a. INQUIRY (if REPORT LUNS supported)
     b. READ CAPACITY (if block device) Handle LongLBA has above.
     c. TUR (START UNIT if not Ready, delay some arbitrary amount of
        time (which is problematic) and go back to TUR.)

Has there ever been a recommended LU discovery sequence in any T10 command
related standard? If yes, I have missed it, please send me a pointer.
Including this information in a standard I belive will improve the quality
of vendor implementations (i.e. reduce interoperability problems).

-- 
Bob Frey
rtfrey at ieee.org

On Tue, Sep 24, 2002 at 06:24:19PM -0600, Pat LaVarre wrote:
> * From the T10 Reflector (t10 at t10.org), posted by:
> * "Pat LaVarre" <LAVARRE at iomega.com>
> *
> Accordingly, the LongLBA bit should dig out who among us has sloppily hard coded op x25 = expect to copy 0 or 8 bytes In.
>  
> The "really large" capacity necessary to exceed 32 bits of Lba space is pretty small these days e.g. 2 TiB at the 0.5KiB/ block common for peripheralDeviceType = x00 DASD.
>  
> Pat LaVarre
> 
> 	-----Original Message----- 
> 	From: Gerry.Houlder at seagate.com [mailto:Gerry.Houlder at seagate.com] 
> 	Sent: Tue 9/24/2002 3:58 PM 
> 	To: t10 at t10.org 
> 	Cc: 
> 	Subject: Re: LongLBA bit in READ CAPACITY (10) command
> 	
> 	
> 
> 	* From the T10 Reflector (t10 at t10.org), posted by:
> 	* Gerry.Houlder at seagate.com
> 	*
> 	
> 	The LONGLBA bit indicates the format of the returned data. If the bit is
> 	zero, the returned data has 4 bytes for max LBA and 4 bytes for the block
> 	size. If the bit is one, the returned data has 8 bytes for max LBA instead
> 	of only 4. it is intended to go with the 8 byte LBA versions of read,
> 	write, etc. on targets with really large capacities.
> 	
> 	
> 	
> 	                                                                                                 
> 	                    "Ken Craig"                                                                  
> 	                    <kcraig at istor        To:     <t10 at t10.org>                                   
> 	                    .com>                cc:                                                     
> 	                    Sent by:             Subject:     LongLBA bit in READ CAPACITY (10) command  
> 	                    owner-t10 at t10                                                                
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> 	                    09/24/02                                                                     
> 	                    04:30 PM                                                                     
> 	                                                                                                 
> 	                                                                                                 
> 	
> 	
> 	
> 	
> 	* From the T10 Reflector (t10 at t10.org), posted by:
> 	* "Ken Craig" <kcraig at istor.com>
> 	*
> 	Can someone tell me what the definition/purpose
> 	of the LONGLBA bit is in the READ CAPACITY (10)
> 	command defined in SBC-2 rev 7?
> 	
> 	Thanks,
> 	Kenneth Ray Craig, Jr.
> 	*
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