Can same physical storage (user data) be accessed through two different LBAs ?

Black, David david.black at emc.com
Thu Apr 23 20:33:33 PDT 2015


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The question is effectively: does SCSI (SBC in this case) prohibit accessing
the same stored logical block data in a read/write fashion via two different
LBAs in the same logical unit?
Write access is crucial here - if one removes the word 'write' then there is
clearly no prohibition, as block-level deduplication cannot be detected by
read/write accesses.  Other mechanisms are necessary to detect that, e.g.,
see subclause 4.8 in SBC-4 rev 5.  Returning to the original question:
> I believe it does because the SBC model clause explains that user data is
in a Logical Block and that a unique LBA maps to the Logical Block.
> But there are no shalls in this explanation.	And I want to be sure all the
explanations about physical blocks and as such do not override this.
The referenced text effectively prohibits a single logical block having two
LBAs in the same logical unit.	However, it does not prohibit writes to one
logical block having the side effect being copied to a second logical block,
which would involve means outside the scope of the SCSI SBC standard.
OTOH, while not prohibited, doing so is a rather bad idea in practice, as
there a quite a bit of server software that uses SCSI storage and makes the
common assumption that for a logical unit that is not shared among servers,
two reads to the same LBA without an intervening write return the same data. 
That assumption does not hold when a write to one LBA affects the results of
a read from a different LBA on the same logical unit without an error being
returned.
> The issue is really to do with access to the same user data rather than the
actual storage location.  The question was over security whether
> if restricting access to a LBA in turn restricts access to that user data.
As has already been pointed out, there are cases in which logical block data
is accessible via multiple logical units.
Thanks,
--David
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Nadesan
Narenthiran
Sent: Thursday, April 23, 2015 9:49 PM
To: T10 Reflector
Subject: Can same physical storage (user data) be accessed through two
different LBAs ?
Hi,
Does SCSI explicitly prohibit accessing the same physical storage location
through two different handles / LBAs (on drives - single LUN devices) ?
The issue is really to do with access to the same user data rather than the
actual storage location.  The question was over security whether if
restricting access to a LBA in turn restricts access to that user data.
This is an off-shoot from a conversation in another reflector.	In that other
conversation I got educated that SCSI does not prohibit access to the same
physical location through two different LUNs and that implementations exist
that use this flexibility.  So I want the answer restricted within a single
LUN.
Also ignore other cases such as copy managers, replication, etc, etc, etc
existing within this device that can copy the data over.
I believe it does because the SBC model clause explains that user data is in
a Logical Block and that a unique LBA maps to the Logical Block.  But there
are no shalls in this explanation.  And I want to be sure all the
explanations about physical blocks and as such do not override this.
If it does prohibit such access, please point out the place(s) that prohibit
this access.
Have a nice day.
Naren.



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