14-176r0 - Including SIZE OF UNMAP REGION in UNMAP CDB

Gerry Houlder gerry.houlder at seagate.com
Thu Jul 3 07:30:07 PDT 2014

Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1407030_f.htm">HTML-formatted message</a>

I am wondering what problem this will solve.
The drive can currently report, in VPD page B0h (Block Limits), the Maximum
Unmap LBA Count and Maximum Unmap Descriptor Count. The drive should be
able to accept any count that is less than these values or else the drive
is reporting a count that is too high. If a host takes notice of the VPD
page values and stays within the limits, then there should be no need for
this additional check (which the drive has to do by transferring all of the
descriptors and calculating whether the lengths of all of the descriptors
matches the new parameter).
The drive can do this instead -- calculate the length of all descriptors
and compare it to the maximum Unmap LBA count in the VPD page instead. This
doesn't require any new parameters.
Are you concerned about catching a rogue host that isn't paying attention
to the VPD page values and sends unmap lengths that are too large anyway?
Those hosts will just put zero in the size of unmap region field anyway, so
adding this check won't help with that.
On Wed, Jul 2, 2014 at 4:33 PM, Dave Landsman <Dave.Landsman at sandisk.com>
>  It is advantageous in certain situations if a device knows the total
> length of a region being requested to be unmapped, before having to process
> the parameter data which describes the UNMAP data in detail.
> An enhancement to the UNMAP command is proposed.  Please see
> http://www.t10.org/cgi-bin/ac.pl?t=d&f=14-176r0.pdf.
> dave
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