FCP-4: Obsolete EMDP bit and data overlay

Robert Snively rsnively at Brocade.COM
Tue Jul 11 09:19:44 PDT 2006

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	It is said that EMDP is useful for offering data that is being
buffered from multiple
	non-synchronized sources (like RAIDed disk drives) as quickly as
	Whichever blocks of data are accessible first are delivered
first, reducing average
	latency significantly.
	I believe that, even if the mechanism is not yet used, high
performance block
	storage devices may eventually like this function and that it
should not be 
data overlay
	It is said that data overlay may be useful for immediate
delivery of buffered
	data that is later found to be incorrect and must be modified.
This behavior may
	appear on tape drives.	The concern about problems with
verifying that all the
	data has been delivered to the correct location may be
manageable through
	the improved data integrity features being provided now in SCSI.
It may also
	be managed through various types of software managed check sums.
	I believe that, even if the mechanism is not yet used, this
	capability should not be removed from FCP-4.
I don't believe that any harm comes from leaving both of these in FCP-4.
In the
future, there may be significant benefits from exploiting these
functions.  At present,
they are clearly optional and may be left out of any implementation.
Usage of these
optional functions is negotiated early in the initialization cycle of
the SCSI devices.
Both have default indicators of '0', making it easy to perform that
negotiation, even
if the SCSI device is not aware of the possible existence of such
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of David
Sent: Monday, July 10, 2006 7:09 PM
To: t10 at t10.org
Subject: FCP-4: Obsolete EMDP bit and data overlay
At the last FCP-4 working group meeting the use of Enable Modify Data
Pointers (i.e., data overlay) was questioned. I myself have never see
the use of data overlay in the FCP realm (other than via FCP-2+ error
recovery which is strictly not considered data overlay).
Please respond if you object to obsoleting the EMDP bit and data overlay
(other than error recovery data overlay) in FCP-4 along with your
(no disclaimer)

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