SBC question about block coherency

Knight, Frederick Frederick.Knight at netapp.com
Mon Nov 14 22:37:45 PST 2011


Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1111150_f.htm">HTML-formatted message</a>

I would also add, that your statement about "the LBA" is also correct.
If in your example, these READ and WRITE commands are operating on an
overlapping multi-LBA range, I'm not aware of any language that requires
serial command completion.
Obvious, as Gerry stated, restricted reordering (see SAM) does add
requirements on WRITES, but multiple normal SIMPLE tagged commands in
the unrestricted environment may all overlap.  If you extend your
READ/WRITE question so that each request involves multiple LBAs, you can
see that "the LBA" is now significant, in that the READ (of multiple
LBAs) is no longer guaranteed to read all old data or all new data -
each LBA would independently be old or new, such that your data-in
buffer may contain some random mix of blocks containing old data, and
blocks containing new data.
I would call it Option 3:
There is an inherent race condition so that as	the WRITE and the READ
commands are processed, they do atomic operations individually on each
LBA referenced by that command, at the same time that the other command
also does atomic operations individually on each LBA referenced by that
command.  No matter which command begins processing first, the data that
is read may contain some whole blocks that contain old data and some
whole blocks that contain new data.
		Fred
From: Gerry Houlder [mailto:gerry.houlder at seagate.com] 
Sent: Monday, November 14, 2011 4:51 PM
To: T10 Reflector
Subject: Re: SBC question about block coherency
Option 1 is the expected behavior. From the point of view of the SCSI
target device, commands are not received simultaneously; there will
always be a mechanism that will cause one of the commands to be logged
as being received ahead of the other.
However there can be other influences on that ordering.
(a) Due to system configuration details (e.g., expanders), command might
be sent from a host in one order but received by the target in a
different order.
(b) If all commands are SIMPLE task attribute, they may be reordered
once they are received into the target's queue. I would not expect
commands that access the same LBA to be reordered with respect to each
other, but this is allowed if "unrestricted reordering" is set in the
Control mode page. If "restricted reordering" is set in the mode page
instead, then reordering of writes with respect to reads of the same LBA
is not allowed.
(c) A command with HEAD OF QUEUE attribute is usually placed ahead of
other commands that are in the queue. There are exceptions for commands
that have already started processing so there is some gray area here.
On Mon, Nov 14, 2011 at 11:32 AM, Kevin D Butt <kdbutt at us.ibm.com>
wrote:
I have a question that seems obvious, but since I come from the tape
world and have not spent much time in the disk world I could be assuming
behaviors that I shouldn't. 
In the tape world, if a logical block is overwritten, then the read of
that logical block cannot occur until after the write has been
completed. The tape's command queue is essentially an ordered queue. In
the disk world, as I understand it, many commands can be processed in
parallel, that is, the queue is not necessarily an ordered queue. So, I
have a question about what that parallel'ness means when the command
arrives with a task type of HEAD OF QUEUE. 
In the example of a WRITE being issued at the same time as a READ being
issued for the same LBA from multiple application clients (i.e., in
different task sets), what should be expected?
Option 1:
There is an inherent race condition so either the WRITE or the READ
command will arrive first and be processed as an atomic operation on the
LBA, then the other command will be processed on the LBA. If the READ
arrives first the data that is read is the old data. If the write
arrives first the new data is written and then the read reads the new
data.
Option 2:
There is an inherent race condition so either the WRITE or the READ
command arrives first, but both commands are processed simultaneously
and the READ command returns data that contains partially old data and
partially new data.
Thanks,
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
Data Protection & Retention
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Tel: 520-799-5280
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at us.ibm.com
http://www-03.ibm.com/servers/storage/ 



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