When your system is initialized during IPL, catalogs are assigned as eligible for either the in-storage catalog cache or the catalog data space cache. These two caches have different performance benefits. Although a catalog might perform well in one type of cache under most circumstances, you might find occasion to remove a catalog from cache, or to move it to a different type of cache. If a catalog using the in-storage cache (ISC) receives a lot of update activity, you might want to remove it from cache until the update activity is finished.

You can move any catalog in the catalog data space to the in-storage catalog cache. Once removed from the catalog data space, a catalog automatically begins using the in-storage catalog cache, unless you use the NOISC parameter to prevent it. If you use the NOISC parameter, the catalog is not cached.

The objective of in-storage cache is to cache only those records that are read directly. Records will not be cached for other types of requests, and therefore in-storage cache is not then in effect. The ISC flag will be turned on the first time a direct read is done. This flag will remain on until that record is no longer in cache. At that point it will remain off until the next direct read to the catalog.

Since the COFVLFxx member of SYS1.PARMLIB controls which catalogs use the catalog data space cache, under normal conditions you cannot move a catalog from the in-storage catalog cache to the catalog data space. If a catalog is entered in the COFVLFxx member, and you have used MODIFY CATALOG,NOVLF to prevent it from using the catalog data space, then issuing the MODIFY CATALOG,VLF command returns the catalog to using the catalog data space.

To change a catalog from ISC to VLF cache management:

1. Update the COFVLFxx parmlib member to add the catalog EMAJ name to the IGGCAS class and recycle VLF
2. Issue F CATALOG,CLOSE(cat)
3. Issue F CATALOG,NOISC(cat)
4. Issue F CATALOG,VLF(cat).

To change a catalog from VLF to ISC cache management:

1. Issue F CATALOG,CLOSE(cat)
2. Issue F CATALOG,NOVLF(cat)
3. Issue F CATALOG,ISC(cat)
4. To make this change permanent, remove the catalog EMAJ name from the IGGCAS class in the COFVLFxx parmlib member, and recycle VLF.

Besides moving a catalog from one cache to the other, you can simply remove the catalog from cache. For example, if you determine that a catalog is not effectively using the catalog data space cache, you can remove it from the cache. The MODIFY CATALOG,REPORT,CACHE command provides information which you can use to evaluate cache performance.

Six parameters of the MODIFY CATALOG command can be used to modify how a catalog uses cache: ISC and NOISC; VLF and NOVLF; and ALLOCATE with NOISC or NOVLF. Which version you use depends on whether the catalog is already allocated to CAS.

If the catalog is not allocated to CAS, then you can allocate it to CAS with the MODIFY CATALOG,ALLOCATE command. The NOISC or NOVLF parameter can be used to prevent the catalog from using the specified cache. Otherwise, the catalog uses the last cache assigned to it (during IPL or a previous MODIFY CATALOG command).

If the catalog is already allocated and using ISC, the NOISC parameter can be used to terminate the ISC. Similarly, NOVLF can terminate the catalog data space.

If you want to assign a catalog in the CDSC to use ISC, close or unallocate it and use the NOVLF option. NOISC must not have been previously specified.

Removing catalogs from the ISC cache also reduces the amount of storage used by the catalog address space, probably at the expense of catalog performance.