SSC-4: Medium partition mode page and differences between mode select and mode sense

Kevin D Butt kdbutt at
Tue Nov 24 11:52:53 PST 2009

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Tape guru's,
I have been looking at the Medium Partition mode page (11h) in SSC-4.  In 
the definition of PARTITION SIZE fields I am trying to determine what the 
difference is between these fields in a Mode Select command vs. a Mode 
Sense command. 
The text that follows in magenta seems to allow a less than desired 
implementation.  In fact, it seems to conflict the intent of Note 70.  If 
the device server is reporting sizes of partitions, why would it not chose 
to set the PSUM and PARTITION UNITS fields such that the partition sizes 
can be represented?  I think that when the PARTITION UNITS field was 
added, this sentence should have been removed.
I have attempted to modify the text from SSC-4 by adding the text in green 
(and suggesting that the text in magenta should be deleted).
PARTITION SIZE fields within the partition size descriptor list define the 
approximate size of the respective partitions in the units specified in 
the PSUM and PARTITION UNITS fields. Partitions are numbered by their 
relative position in the partition size descriptor list, starting at 0. 
Only partition numbers in the range of 0 to n where n is less than or 
equal to 63 may have size descriptors in this mode page. Partition n, if 
present, shall be described by the partition size descriptor at mode page 
offsets 8+(2*n) and 9+(2*n). Partition 0 shall be the default partition. 
Partition size descriptor 0, shall contain the size of the default 
partition. The size of partition 0 shall be greater than 0. Up to 64 
partitions may be defined using this mode page. Partitions not assigned 
shall have a partition size descriptor of 0. The logical unit may support 
more partitions than partition size descriptors. A logical unit may 
support more partition size descriptors than supported by the volume. All 
partition size descriptors representing a partition number greater than 
the maximum additional partition count shall be 0. The partition size 
descriptors are undefined if the logical unit is not ready. A MODE SELECT 
command partition size descriptor of FFFFh requests that the logical unit 
allocate all remaining partition space to that partition. A MODE SENSE 
command shall return a partition size descriptor of FFFFh if the partition 
size, in units of PSUM or PARTITION UNITS, is greater than or equal to 
FFFFh. If insufficient space exists on the volume for the requested 
partition sizes or if multiple partition size descriptors are set to 
FFFFh, the device server shall return CHECK CONDITION status. The sense 
key shall be set to ILLEGAL REQUEST and the additional sense code shall be 
set to INVALID FIELD IN PARAMETER LIST. A device server may round, as 
described by the MODE SELECT command in SPC-4, any partition size to the 
nearest valid partition size. When a MODE SENSE command is received after 
a MODE SELECT command that had the POFM bit is set to zero but before a 
FORMAT command the values in the PARTITION SIZE, PSUM, andPARTITION UNITS 
fields shall be the values set by the MODE SELECT command. When a MODE 
SENSE command is received, the logical unit is ready, and there is no 
pending change related to a MODE SELECT command for this page the PSUM, 
and PARTITION UNITS fields and any PARTITION SIZE fields that are returned 
shall reflect the size on the medium of the related partition.
NOTE 70 - It is recommended, but not required, that the number of 
partition size descriptors available through the Medium Partition mode 
page equal at least the number of maximum additional partitions + 1. This 
provides a mechanism for the device server to disclose the current 
partition sizes.
Does this sound correct and does it look like something that we should 
attempt to clarify in SSC-4. If the answer is yes, I will prepare a 
proposal to explicitly state the difference between Mode Select and Mode 
Sense with and without a pending change.
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Tel: 520-799-5280
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at

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