Removable Medium Problems
jojo at cc2smtp.tdata.no
Thu Jan 25 12:50:50 PST 1996
* From the SCSI Reflector, posted by:
* JOJO <jojo at cc2smtp.tdata.no>
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 126.96.36.199).
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
Data Compression Page
We have two questions regarding the Mode Select Data Compression Page
specified in X3T10/997D rev.5 page 48 (section 188.8.131.52).
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 tdata.no
TANDBERG DATA STORAGE AS
P.O Box 134 Kjelsas
N-0411 Oslo, NORWAY
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