The user can influence storage space allocation on public disks by means of the SPACE operand of the FILE macro or the CREATE-FILE and MODIFY-FILE-ATTRIBUTES commands.
If the SPACE operand is not specified when a file is cataloged by means of the FILE macro or CREATE-FILE command, the system default values are used. If a file is cataloged by means of the CATAL macro, only the catalog entry is created, i.e. no space is allocated to the file.
The SPACE operand defines the primary and/or secondary allocations.
Storage space requests of up to 4 TB are possible. This is the maximum capacity of a pubset minus pages used for the management data of the system (TSOSCAT, user catalog, F5 label...). The theoretical maximum file size that can be represented in the catalog is
2,147,483,647 PAM pages.
Primary allocation
This storage space is immediately reserved or released for the file. The value specified in the SPACE operand is rounded up to a multiple of “k”.
(“k“ being the number of 2-Kbyte blocks per allocation unit, the smallest memory management unit; see "Requesting storage space").
If the value in the macro is positive or is specified in the commands with the operand SPACE=*RELATIVE or SPACE=*ABSOLUTE, DMS reserves the corresponding number of PAM pages for the file.
If the value in the macro is negative or is specified in the commands with the SPACE=*RELEASE operand, the corresponding number of free PAM pages are released, provided this is possible.
Secondary allocation
In contrast to the primary allocation, the secondary allocation has no effect when a file is created; it is only used if the reserved storage space is insufficient when creating or extending the file. The system then automatically increases the space allocation for the file by the number of PAM pages defined as the secondary allocation (SHOW-FILE-ATTRIBUTES command, output field S-ALLOC).
The value for the secondary allocation is doubled after each successful extension. Doubling the value is discontinued when the maximum value set in the system is reached. If required, the defined number of PAM pages is rounded up to a multiple of “k” when allocating space. This process may be repeated until the total storage space available to the user is exhausted.
Allocation of contiguous storage areas (extents)
For both primary and secondary allocation of storage space, DMS attempts to reserve contiguous storage areas and to avoid splitting a file over several disks. The resulting file sections are called extents. The FSTAT macro and the SHOW-FILE-ATTRIBUTES
command show the user how many extents there are in a file and on which volumes or disks these extents are stored.
Files on Net-Storage always have (only) one extent.
Storage space is reserved in “allocation units”, the smallest memory management unit. An allocation unit comprises a specific number of 2-Kbyte blocks; currently available sizes are 6K, 8K and 64K. Systems support can define the size of an allocation unit at pubset generation, subject to certain restrictions depending on the disk format. Pubsets including key disks, for instance, accept 6K units only.
The table below shows the interdependencies of rounding factor k, allocation unit size and corresponding number of PAM pages:
“k” | Unit size in Kbytes | Number of PAM pages |
|---|---|---|
6 | 6 | 3 |
8 | 8 | 4 |
64 | 64 | 32 |
File processing speed can be increased by preventing files from being distributed over too many extents. For instance, files can be reorganized by copying (COPFILE macro or COPY-FILE command).
Physical allocation
Physical allocation is the explicit specification of the storage location for a file; this can be done in any of the following ways:
specification of a volume set
specification of a volume
Absolute allocation
This means specifying which and how many PAM pages are to be reserved for a file by entering the start address and the size of the area to be reserved in the SPACE operand.
Absolute allocation is not possible for Net-Storage volumes.
Types b) and c) of physical allocation may generally be used for private disks. For pubsets, physical allocation can be used in the following cases:
for files on public volumes, provided the pubset was made available with the appropriate attribute (see output of the command SHOW-MASTER-CATALOG-ENTRY ...,INFORMATION=*USER)
for files on public volumes, provided the user has been granted the physical allocation right (see output of the command SHOW-USER-ATTRIBUTES ...,INFORMATION=*PUBSET-ATTRIBUTES).
for work files in SM pubsets.
Insufficient storage space
If the system cannot satisfy a request for storage space, it proceeds as follows:
either the macro or command issued for primary allocation is rejected or only part of the requested space is allocated
in the case of secondary allocation, the program may be aborted.
DMS rejects a space request in the following cases:
The request would exceed the public space limit available to the user on this pubset and the user has no authorization to exceed his/her public space limit (PUBLIC-SPACE-EXCESS=NO in the user entry).
Because there is insufficient free space on the pubset or private disk in question, further allocation of storage space is not possible.
The request was submitted for Net-Storage and the use of Net-Storage is not permitted in the user entry (NET-STORAGE-USAGE=*NOT-ALLOWED). By default the use of Net-Storage is permitted, but no limit can be specified.
If the new request would make the number of necessary extents too large for the extent list in the catalog entry or if the space request exceeds the free capacity of a private disk, DMS executes a partial allocation.
If a request for storage space would exceed the maximum file size that can be represented in the catalog entry, only the maximum possible partial allocation is made.