To : X3T9.2 Committee X3T9.2/90-130 From : Steve Krupa, HP CPB Date : 16th August 1990 Subject : Response to committee document X3T9.2/90-116 In response to the interpretation proposed to X3T9.2 for the Partition Size Descriptor field(s) in the Medium Partitions Page(s), (document X3T9.2/90-116) we submit the following comments: Mode Sense/Select Inconsistencies and Technology issues ======================================================= 1. For a Mode Sense command, if the tape contains only a single partition (partition 0) then, according to the interpretation the Medium Partitions Page is 6+2 bytes long and contains 1 Partition Size Descriptor which contains a number respresenting the size of the media (or an estimate of it) currently in the drive. 2. For a Mode Select command, if the tape is to be formatted to contain a single partition then, according to the interpretation, the Medium Partition Page is 6+2 bytes long and contains 1 Partition Size Descriptor which contains a number representing the size of the media (or an estimate of it) currently in the drive. In this case the IDP bit will be set to 1 to indicate that the initiator is defining the partitioning parameters. However, the Additional Partitions Defined field is by necessity 0, indicating that the tape should be formatted as a single partition. In this case the Partition Size Descriptor field is completely meaningless, firstly because the device will reformat the media so that all of its available length is in partition zero, and secondly because in section 9.3.3.2 on Medium Partition Page (1), SCSI-2 Rev 10c states that .. "An initiator-defined partitions (IDP) bit of one indicates that the initiator is defining the number and size of the data partitions using the additional partitions defined field." which appears to preclude the use of the Partition Size Descriptor if the Additional Partitions Defined field is 0. 3. Paragraphs 1 and 2 are therefore inconsistent in the definition of the use of the Partition Size Descriptor field. 4. For a Mode Sense command, if the tape contains N partitions, then according to the interpretation the Medium Partitions Page is 6+2N bytes long and contains N Partition Size Descriptors which contain numbers representing the size of each partition (or estimates of them) on the media currently in the drive. The sum of the representative numbers is equal to the total size of the media. 5. For a Mode Select command, if the tape is to be formatted to contain N partitions then, according to the interpretation, the Medium Partitions Page is 6+2N bytes long and contains N Partition Size Descriptors which contain numbers representing the size of each partition (or estimates of them) required to be laid down on the media currently in the drive. The sum of the numbers is expected to equal the maximum capacity of the medium. Due to the very nature of some sequential-access storage technologies, including 3.81mm helical-scan technologies, such a sum cannot be computed with any useful degree of accuracy, even if N is no greater than 2. Overhead is required in order to fulfil logical format obligations and also to ensure that that enough space is available to take account of repeated groups and frames, e.g. as caused by Read-After-Write check failures. Each vendor may do this differently. It is not simply a matter of taking the total media capacity as reported in a previous Mode Sense command and splitting it in 2. Which of the N partition sizes (2 in this case) requested by the initiator should be taken as being correct by the device and which should be ignored as being there "just to complete the Medium Partitions Page" ? 6. Paragraphs 4 and 5 are therefore inconsistent in the definition of the use of the Partition Size Descriptor field. 7. An alternative proposal is to allow the use of the Partition Size Descriptor fields to differ between Mode Sense and Mode Select commands. For the Mode Sense command, Partition Size Descriptors would be returned for ALL partitions on the current tape. For the Mode Select command, Partition Size Descriptors would be defined for all ADDITIONAL partitions on the current tape, with any remaining capacity being assigned to the default data partition. This proposal has been made by Archive Corporation. 8. In general, if a host requires that a target device changes its operating mode or configuration (via a Mode Select command), it will firstly issue a Mode Sense command to read the current target mode, modify just the parameters it wishes to change and then issue a Mode Select command and write the new target mode back to the target device. It should not be the case that the host is also required to change the SIZE of the Mode Page that it is rewriting to the target. The host is only interested in the parameters which directly affect the target operating mode and should not have to concern itself with other details. Because of this, the alternative proposal outlined in paragraph 7 is inconsistent with the general usage of the Mode Sense/ Select commands. Conclusion ========== Hewlett-Packard disagrees with the proposal given in document X3T9.2/90-116 and does not support the alternative proposed by Archive Corporation. Instead, we propose that the Partition Size Descriptor field in the Medium Partition Page is limited to Additional Partitions Defined only. We therefore propose that the document X3T9.2/90-116 be not ratified by X3T9.2, and request that the committee allow time for discussions between Hewlett-Packard, Archive and other interested parties in order to develop a better solution to this problem. We would aim for this to be complete before the October plenary. We believe that this extension of time is reasonable, and offers a better solution taking a hasty, but unsatisfactory, decision now. Steve Krupa Pete Bramhall Hewlett-Packard, Computer Peripherals Bristol, Filton Rd, Stoke Gifford, Bristol, BS12 6QZ England Tel : + 44 272 799910 Fax : + 44 272 236091