SPC-4 question on disconnect-reconnect mode page parameter

Knight, Frederick Frederick.Knight at netapp.com
Mon Jan 26 07:47:04 PST 2009


* From the T10 Reflector (t10 at t10.org), posted by:
* "Knight, Frederick" <Frederick.Knight at netapp.com>
*
My opinion is that this information existed pre-DIF, and was not updated
when protection information was added to SBC.  Yes, time makes more
sense, but the values are already specified as 512-byte block based, not
time based.
So, I would suggest that the proper interpretation is to read the word
"data" as "user data".	The problem is that "user data" is defined in
SBC, not in SPC, but here is the idea:
The MAXIMUM BURST SIZE field indicates the maximum amount of <user> data
that the
target port shall transfer during a single data transfer operation. This
value is expressed in increments of 512 <user data> bytes (i.e., a value
of one means 512 bytes <of user data>, two means 1 024 bytes <of user
data>, etc.). The relationship, if any, between data transfer operations
and interconnect tenancies is defined in the individual SCSI transport
protocol standards. A value of zero specifies there is no limit on the
amount of <user> data transferred per data transfer operation.
The issue of course, is that "user data" is defined in SBC (and relates
to protection information that is defined in SBC), but the above text is
|from SPC (where "user data" is not defined, and there is no concept of
protection information).
Another choice might be to add something in SBC that further clarifies
the SPC text, and more specifically states that the value in that field
represents "user data".  Or, in SBC, you could say:
If protection information is enabled, then this value is expressed in
increments...
If protection information is not enabled, then this value is expressed
in increments...
Either way, it sounds like we'll be seeing a proposal at the next
meeting to suggest how to deal with this issue. 
	Fred Knight
	NetApp
	SAN Standards Technologist
-----Original Message-----
From: Gerry.Houlder at seagate.com [mailto:Gerry.Houlder at seagate.com] 
Sent: Friday, January 23, 2009 2:02 PM
To: t10 at t10.org
Cc: Narayan.Ayalasomayajula at emulex.com
Subject: Re: SPC-4 question on disconnect-reconnect mode page parameter
* From the T10 Reflector (t10 at t10.org), posted by:
* Gerry.Houlder at seagate.com
*
Most people think of the maximum burst size in terms of how long a
system
is willing to dedicate bandwidth to a particular task. The time concept
is
easier to keep in focus when it is related to bytes transferred, not a
multiple of logical blocks that can vary in size.
	     Bill.Martin at emule
	     x.com
	     Sent by:
To 
	     owner-t10 at t10.org	       <t10 at t10.org>
cc 
<Narayan.Ayalasomayajula at emulex.com 
				       >
	     01/21/2009 04:46
Subject 
	     PM 		       SPC-4 question on
				       disconnect-reconnect mode page
				       parameter
In looking at the disconnect-reconnect mode page in SPC4r17, the
definition
for the MAXIMUM BURST SIZE is:
The MAXIMUM BURST SIZE field indicates the maximum amount of data that
the
target port shall transfer during a
single data transfer operation. This value is expressed in increments of
512 bytes (i.e., a value of one means 512
bytes, two means 1 024 bytes, etc.). The relationship, if any, between
data
transfer operations and interconnect
tenancies is defined in the individual SCSI transport protocol
standards. A
value of zero specifies there is no limit
on the amount of data transferred per data transfer operation.
This works for devices that formatted for data in multiples of 512 bytes
per block; however, this does not map well for devices that formatted
for
Protection information and the block size is 520 bytes.  I understand
that
the data transfer length can be easily compared to number of 512 byte
blocks; however, if you want to have a max burst size of 32 blocks that
are
520 bytes each then this field does not easily map to the number that
you
want because 33 would map to 32.49 520 byte blocks.  Was there any
thought
about tying this to the block size that a storage device was formatted
for?
Thanks,
Bill Martin
Emulex
Office of Technology
Industry Standards
916 772-3658
916 765-6875 (Cell)
bill.martin at emulex.com
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