User requirements with regard to performance, availability and file structure are specified via the file attributes already used with SF pubsets.
The user command SHOW-PUBSET-FILE-SERVICES enables users to make efficient use of the benefits of an SM pubset. It lists the services that are logically available in an SM pubset. It provides information about what combinations of the file attributes AVAILABILITY, FORMAT, PERFORMANCE, USAGE, DISK-WRITE (and WORK-FILE) which are relevant to the storage location and that are possible on an SM pubset, about impossible combinations, and about combinations that are possible but cannot be optimally implemented. The determination of available services takes neither the current occupancy rate of individual volume sets nor user authorizations or user contingents into account.
The following file attributes relevant for file location are used to specify user requirements:
Preformat
The meaning of the structure attributes (K, NK2, NK4) is the same for SM pubsets and SF pubsets. However, the mechanisms used to assign structure attributes to a file are different. One feature of SM pubsets that is particularly important is that information about the file format can be supplied at the time storage space is reserved for a new file or a file that has already been cataloged but not yet been allocated any storage space. So that the system can take this information into account when selecting the volume set. The format information is supplied via the file attribute PREFORMAT, which is assigned to a file at initial storage space allocation via the commands CREATE-FILE and MODIFY-FILE-ATTRI-BUTES (FILE-PREFORMAT operand). (N.B.: “Initial storage space allocation” includes the allocation of storage space to a file that has been cataloged but not yet allocated any storage space.) For files created via the FILE macro, the preformat is derived, among other things, from the user specification for BLKCTRL and BLKSIZE made at file creation (i.e. at the time of storage space allocation). The term “preformat” indicates that this is a preliminary format. The final file format is not defined until the file is opened for creation. The volume set on which storage space has been reserved for the file may be found to be incompatible with the desired file format at OPEN time. In this case, OPEN processing implicitly relocates the file to a suitable volume set, if available. To keep system overhead low and to avoid the risks involved in relocation (memory bottlenecks etc.), it is highly advisable to ensure that a suitable preformat is specified for files of an SM pubset at the time storage space is allocated. If no preformat is explicitly specified for a file, a default preformat is assumed. (For information on assigning a default, see section "Defaults forlocation-relevant file attributes".)
Performance
The file attributes PERFORMANCE, USAGE, DISK-WRITE for SF pubsets are also used to specify performance requirements for files on SM pubsets. However, these performance attributes have a more general meaning with SM pubsets, since they then not exclusively refer to cache utilization. The initial effect of the attributes statically assigned via the commands CREATE-FILE and MODIFY-FILE-ATTRIBUTES is that a suitable volume set is selected at file creation. This is not necessarily a volume set equipped with a cache because there are other means to improve performance. The statically assigned values may be reduced for current file processing via dynamic modification of the performancerelevant file attributes PERFORMANCE and USAGE. This has no effect on the location of a file; it only refers to the utilization of the cache of the volume set where the file resides at OPEN time. If the volume set has no cache, dynamic modification of the performance attributes has no effect.
Availability
The file attribute AVAILABILITY was introduced to specify to what extent a file is expected to be failsafe (CREATE-FILE and MODIFY-FILE-ATTRIBUTES commands, AVAIL operand in the FILE and CATAL macros). In contrast to the performance attributes, this specification has a binding effect, i.e. the attribute is guaranteed. The specification AVAILABILTY=*HIGH is rejected for work files and temporary files. It is likewise rejected if no suitable volume set is available or if the contingents assigned to the user ID are exceeded. If AVAIL-ABILTY=*HIGH is requested for a file that is already occupying storage that only meets AVAILABILITY=*STD requirements, the file is relocated to a suitable volume set.
Work file
Work files are files that are assigned the file attribute WORK-FILE.
This is not possible unless a work volume set exists on the SM pubset (see the SHOW-PUBSET-DEFINITION-FILE command). A description of work files is given on "Work files".
Prospective file size
The prospective file size influences the selection of a suitable volume set with regard to the allocation unit size. It cannot be explicitly specified by the user and is deduced from the values specified for the primary and secondary allocation instead. Users can simply specify whether or not their files may grow to become so-called “large files” (file size >= 32 GB) (see "Files larger than 32 GB").
Validity of location-relevant file attributes
The file attributes performance, availability, preformat and work file, which are relevant for the location of a file, are assigned when the file is allocated storage space for the first time. Both space allocation and attributes remain valid until either modified by the user or until the space allocation is cancelled. If a file is cataloged but not allocated any storage space (not even on any background volumes), the file attributes relevant for the location of the file (with the exception of the performance attributes) are undefined. They are determined on the basis of the user specifications made when the user (again) requests storage space for the file (MODIFY-FILE-ATTRIBUTES command).
Adapting file location in accordance with modified requirements
The MODIFY-FILE-ATTRIBUTES command enables the user to modify the static performance attributes PERFORMANCE, USAGE, DISK-WRITE as well as the AVAILABILITY attribute of an existing file, thus informing the system of modified performance and availability requirements. The preformat cannot be modified. A higher availability cannot be requested unless covered by the service range of the SM pubset. Note that the volume set on which the file is currently residing may prove to be less than optimally suitable for or even incompatible with the new performance or availability attributes.
If the new file attributes are incompatible with the current volume set, the file is implicitly and immediately relocated to a compatible volume set, if available. If the current volume set is no longer ideally suited, the file remains on the volume set, even if user requirements are not fully met or resources not optimally used. Systems support then has to restore optimum distribution of files over the individual volume sets by means of pubset reorganization. Implicit reorganization occurs whenever a file is recalled to the processing level from a background level, since the selection of a volume set in this case is based on the current file attributes.
Being performed at the physical level, the activities involved in file relocation are (largely) invisible to the user. In particular, they have no effect on file addressing and do not require any adaptation of the user JCL.