Relative offset calculation with data protection enabled

Elliott, Robert (Server Storage) Elliott at
Tue Jun 6 11:16:39 PDT 2006

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Protection information is a SCSI Block command set concept.  The Fibre
Channel and FCP layers just transfer bytes for the SCSI layer.	They don't
know if the command they are carrying is block-based or not, what the block
size is, or even what a "block" is.  Some SCSI command sets like OSD (object
storage) and SSC (tapes) have totally different concepts of the granularity
of media accesses than SBC (disks).  So, the Parameter field in the FCP
frame header, which serves as a Relative Offset field for FCP_DATA IUs, is
based on the number of bytes actually transferred (e.g., 520 bytes per
block) not the amount of user data (e.g., 512 bytes per block).
Rob Elliott, elliott at 
Hewlett-Packard Industry Standard Server Storage Advanced Technology 
From: owner-t10 at [mailto:owner-t10 at] On Behalf Of
Gary.Franco at
Sent: Tuesday, June 06, 2006 11:24 AM
To: t10 at
Subject: Relative offset calculation with data protection enabled
I have been trying to find some statement in the various documents defining
data protection, SPC/SBC and have yet to find anything discussing relative
offset generation with a data protection enabled device or initiator.
Is the relative offset field still generated on 512 byte blocks and the DIF
field just an opaque object that we have to deal with?
When a check and translate function is requested (DIF embedded in the
data-in or data-out buffer) does the above still hold true?
If you know where I can find this information I would appreciate a reply.
Gary Franco Consultant Engineer 
Emulex Network Systems 
972-671-7433 Dallas Office
972-671-7435 Dallas Office Fax 
720-652-6387 Longmont Colorado Office 
720-494-1817 Longmont Colorado Office Fax
972-839-5694 Cell Phone
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