BS2000 supports magnetic tape cartridge (MTC) systems with different data formats (number of tracks) and recording methods (with and without compression). The types supported in this version are described in the Release Notice for BS2000 OS DX.
The volume type determines the data format (HSI) and thus implicitly selects the MTC device. With device type 3590E, the volume type can additionally be used to determine the recording method. The volume type table is contained in the "System Installation" manual [15 (Related publications)].
As one would expect, files generated on any of these MTC devices cannot simply be read and further processed on one of the other device types without further ado.For this to be possible, systems support must comply with the guidelines and recommendations outlined below.
Basic prerequisites from the point of view of the system
“Compression” as a volume attribute
In the same way as the data format, “compression” is regarded as a volume attribute, i.e. the files on a tape cartridge are recorded in either compressed or non-compressed form. However, in this context users should bear in mind that all labels are always written without compression, including those belonging to files which contain compressed data.
Compression and data format as customer options
The system does not automatically set a data format and a recording method. It is systems support's responsibility to set the desired values for the system (by means of an appropriate parameter specification in the FILE macro with parameter DEV or CREATE-FILE or IMPORT-FILE command with parameter DEVICE-TYPE).
Internal system considerations mean that the only parameter available for these entries in the FILE macro and CREATE-FILE command is the existing DEV and DEVICE-TYPE parameter, respectively. As a result this therefore defines both the device to be used and the recording method. The full responsibility for the use of the DEV parameter rests with the user. It may be necessary to modify this parameter in existing procedures.
There is no generic term for “MTC” (without defining further options).
Consequently, the user should exercise due caution when specifying the DEV or DEVICE-TYPE parameter, to ensure that his files are processed exactly as required.
Possible entries for the DEV parameter
A number of cases must be distinguished:
Creating a new file as the first file in a multifile configuration
The DEV or DEVICE-TYPE value specified by the user determines the device to be used as well as the desired recording method.
Processing (reading) an existing file
Two cases must be distinguished when reading a file:
There is a catalog entry. In this case, the volume type must be stored in the catalog entry. The system selects a suitable device and the file is read in the correct mode.
There is no catalog entry, i.e. the file is treated as a foreign file. In this case, it is the user's responsibility to ensure that the correct device is selected in the DEV or DEVICE-TYPE parameter. In other words, the user has to know precisely how the data on his magnetic tape cartridges was recorded.
In the case of files with standard labels, DMS checks the compatibility of specifications in the FILE macro or IMPORT-FILE command, or of catalog entries with the values stored in the labels of existing files. Whether or not a file (and thus the entire volume) is compressed is indicated in bytes 34 and 35, which are reserved for BS2000, in the HDR2 (and EOF2) labels.
In the event of a discrepancy between the values entered by the user or stored in the catalog and the information in the label, the system rejects the job.
Extending files and creating continuation files
When extending a partly existing file by means of OPEN EXTEND or INOUT, a check as to whether the entry from the file label matches the desired processing mode ensures that any new data written to the file is likewise compressed.
If a continuation file is created on a multifile tape, the first file on the tape determines the only possible processing mode. The entry in the label of the first file serves to ensure that compression is treated as a volume attribute.
If there are any incompatibilities, the system rejects the job.
Note
All checks performed by DMS can only refer to tapes created with standard labels and supplied with the entry “BS2000” in the HDR2 label. As the compression indicator in HDR2 is not contained in a standard label field, unambiguous identification of the compression status is not possible for tapes not produced by Fujitsu Technology Solutions (or for tapes with nonstandard labels or without labels). When working with tapes without standard labels, it is the user's task to ensure that the correct compression mode is used.