Removable Medium Problems

JOJO jojo at
Thu Jan 25 12:50:50 PST 1996

* From the SCSI Reflector, posted by:
* JOJO <jojo at>
     We have some questions regarding medium partitions and data compresssion. 
     We are looking at a sequential access device (like a tape streamer). In 
     this case the medium is typically removable. It is also typical that the 
     device is able to operate on medias of diffent types. The different medium 
     types have different capabilities (like different capacities, varying 
     possibilites of storing information other then the bare data etc.)
     Medium Partition Page
     We have a question regarding the Medium Partition Page(1) specified in 
     X3T10/997D rev.5 page 54 (section
     The description of the "Maximum Additional Partitions" field indicates that 
     the value of this field is "a logical unit-defined value indicating the 
     maximum number of additional partitions supported by the LOGICAL UNIT".
     In our case however, the maximum number of partitions is determined by the 
     type of the MEDIUM currently inserted into device (a QIC-1000 type medium 
     allows a maximum of 2 partitions while a QIC-5010 type medium allows 36 
     partitions). In some ways this is also more logical in that the both the 
     actual total capacity and the how the total capacity is structured (into 
     partitions) are both typically determined by the medium and not by the 
     device (partitioning is a medium attribute).
     1) Should not the "Maximum Additional Partitions" field be a medium 
     type-defined value (and not a logical unit-defined value)? As it is now it 
     is not possible to determine the maximum number of partitions allowed by 
     the medium?
     Data Compression Page
     We have two questions regarding the Mode Select Data Compression Page 
     specified in X3T10/997D rev.5 page 48 (section
     In our case we have some medium types that do not have the possebility 
     of indicating that the data on the medium have been compressed (and 
     because of this there is no indication of the compression algorthm 
     2) When such a medium is inserted into the device, what should a Mode 
     Sense command report in the Data Compression Page?
     Since there may be possible problems related to using compression on 
     such mediums (like reading back data without knowing if it has been 
     compressed or not), we would like to dissallow using compression 
     altogether on these media types.
     3) How should a device indicate that compression is currently not 
     We have three suggestions, we but are not sure which one is 
       A)  The Data Compression Capable (DCC) field may be used. However, 
       ANSI defines this field as a "non-changable" field. The field is 
       related to the device and not to the currently inserted medium.
       B)  The Data Compression Enable (DCE) field may be used and a Mode 
       Select with this bit set can give Check Condition when the currently 
       inserted medium has a type that we do not want to use for compressed 
       data. However, DCE may be enabled BEFORE the medium is inserted 
       (before we know the medium type).
       C)  The Decompression Algorithm field may be used. If the inserted 
       medium is not blank, the host may give a READ 1 block command and 
       issue a Mode Sense to get the Decompression Algorithm. A 
       Decompression Algorithm of zero indicates no compression (the medium 
       type is such that compression is not allowed). If the inserted tape 
       is blank, a single dummy block may be written (DCE=1) and read again. 
       As above, the Decompression Algorithm may indicate if compression was 
       used (that is if was allowed by the device or not).
     Bjarte Myrold via
     Jorgen Johanson            jojo at
     P.O Box 134 Kjelsas
     N-0411 Oslo, NORWAY

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