Short-on-storage conditions in dynamic storage areas
If the limit for a dynamic storage area (DSA) is too small, the CICS® region periodically enters a short-on-storage condition. Where possible, CICS curtails system activity until it can recover enough storage to resume normal operations. Use CICS messages and statistics to monitor when a short-on-storage (SOS) condition is entered, and when it is relieved.
CICS attempts to resolve pressures on storage before entering a short-on-storage condition. When CICS starts to become short on space in a DSA, the situation is known as a storage stress condition. Where possible, CICS takes actions such as deleting programs that are not being used (program compression), deleting cached copies of any document templates, and searching for free extents in other DSAs. If these actions fail to resolve the storage stress condition, CICS declares an SOS condition for the DSA.
During an SOS condition, CICS takes steps to limit work, so that there is enough storage to process work that is already in progress. CICS prevents acquisition of new input message areas, and defers all ATTACH requests from CICS system modules. Limiting work degrades the performance of the CICS region. In extreme circumstances, an SOS condition might also lead to storage deadlock abends.
- DFHSM0131 for 24-bit storage
- DFHSM0133 for 31-bit storage
- DFHSM0606 for 64-bit storage
SOS conditions are also recorded in the CICS statistics for the dynamic storage area
(Times went short on storage
). You can use the CICS commands CEMT INQUIRE SYSTEM, EXEC
CICS INQUIRE SYSTEM, and CEMT INQUIRE DSAS to
inquire about SOS conditions.
- For 24-bit storage, check whether the limit for the DSAs in 24-bit storage is as high as possible. If required, you can change the DSALIM parameter while CICS is running.
- For 31-bit storage, check whether the limit for the extended dynamic storage area (EDSA) is as high as possible. If required, you can change the EDSALIM parameter while CICS is running.
- For 64-bit storage, check whether there is sufficient 64-bit storage for the CICS region. If required, you can change the z/OS® MEMLIMIT value, but only on the next start of the CICS region.
CICS reserves areas of contiguous virtual storage, called storage cushions, in each DSA. A storage cushion is used only when there is not enough free storage in the DSA to satisfy an unconditional GETMAIN request. In a storage stress condition, the storage cushion might avert a storage deadlock. The CICS storage manager statistics for the dynamic storage areas show the number of times that CICS needed to use storage from the cushion. A request might be larger than all the remaining storage in the DSA, so that even the storage in the cushion is insufficient. When a request is suspended for this reason, the suspension is also shown in the CICS storage manager statistics for the dynamic storage areas.
Short-on-storage conditions for 24-bit and 31-bit storage
When an individual DSA in 24-bit or 31-bit storage, for example the CDSA, requires additional storage, the CICS storage manager allocates another extent to that DSA. Additional extents can be acquired as necessary until the DSALIM or EDSALIM limit is reached, as appropriate. When all the possible extents are allocated, CICS searches for a free extent in another DSA, to relocate it to the DSA in need. For CICS to remove an extent from one DSA so that it can be allocated to another, all pages in the extent must be free. That is, no pages must be allocated to any subpool.
Program compression might be triggered when the DSALIM or EDSALIM limit is approached and there are few free or empty extents available. The DSAs that contain programs are evaluated individually to determine whether program compression is required. In systems with a moderate proportion of loadable programs, program compression is an indicator of pressure on virtual storage.
- No further extents can be allocated or relocated from other DSAs.
- Program compression has been attempted.
- All nonresident programs that are suitable for deletion, and that are not in use, have been deleted.
- All cached copies of document templates that are suitable for deletion. See Caching and refreshing of document templates.
- Storage from the storage cushion is in use (that is, the number of contiguous free pages is less than the number of pages in the cushion), or at least one request is suspended because there is no contiguous area of storage large enough for it, or both of these conditions apply.
Short-on-storage conditions for 64-bit storage
For 64-bit storage, CICS tracks the total amount of 64-bit storage in use for the CICS address space. This storage includes both the above-the-bar dynamic storage area (GDSA), and MVS™ storage in the CICS region outside the GDSA.
CICS considers an SOS condition when storage from the storage cushion is in use, or at least one request is suspended because there is no contiguous area of storage large enough for it, or both of these conditions apply. No further extents can be allocated for a DSA in 64-bit storage if the sum of all allocated above-the-bar storage and the size of a new extent would exceed the MEMLIMIT value.
- Current GDSA active
- Peak GDSA active
- Number of IARV64 CONVERT(FROMGUARD) failures
- Current GDSA allocated
- Peak GDSA allocated
- Times cushion released
- Times went short on storage
An IARV64 CONVERT(FROMGUARD) failure indicates that a request for 64-bit storage has failed. A request might fail because there is not enough auxiliary storage in the system to back the request. Also, a request might fail because a component that the CICS storage manager does not control, for example, a JVM server, has allocated so much storage that the storage manager is affected. CICS cannot resolve pressures on storage caused by components outside the GDSA allocating storage, so you must use the CICS statistics to identify such problems.