ABCs of z/OS System Programming Volume 3 - IBM Redbooks

If you need to share data sets across systems, it is advisable that you share the catalogs that
contain these data sets. A BCS catalog is considered shared when both of the following are
true:
► It is defined with SHAREOPTIONS (3 4).
► It resides on a shared device, as defined at HCD.
Attention: To avoid catalog corruption, define a catalog volume on a shared UCB and set
catalog SHAREOPTIONS to (3 4) on all systems sharing a catalog.
Using SHAREOPTIONS 3 means that VSAM does not issue the ENQ SYSVSAM SYSTEMS
for the catalog; SHAREOPTIONS 4 means that the VSAM buffers need to be refreshed.
You can check whether a catalog is shared by running the operator command:
MODIFY CATALOG,ALLOCATED
A flag in the catalog indicates whether the catalog is shared.
If a catalog is not really shared with another system, move the catalog to an unshared device
or alter its SHAREOPTIONS to (3 3). To prevent potential catalog damage, never place a
catalog with SHAREOPTIONS (3 3) on a shared device.
There is one VVR in a shared catalog that is used as a log by all the catalog management
accessing such catalog. This log is used to guarantee the coherency of each catalog buffer in
each z/OS system.
Guaranteeing current information
For performance reasons, CIs contained in a master catalog are buffered in the catalog
address space (CAS). Optionally (and highly advisable), the user catalogs are buffered in
VLF data space. Thus, before each search in the buffers looking for a match, a simple I/O
operation checking is performed in the log VVR. If the catalog was updated by another
system, the information in the buffer needs to be refreshed by reading the data from the
volume.
The checking also affects performance because, to maintain integrity, for every catalog
access a special VVR in the shared catalog must be read before using the cached version of
the BCS record. This access implies a DASD reserve and I/O operations.
To avoid having I/O operations to read the VVR, you can use enhanced catalog sharing
(ECS). For information about ECS, see 6.24, “Enhanced catalog sharing” on page 375.
Checking also ensures that the control blocks for the catalog in the CAS are updated. This
occurs if the catalog has been extended or otherwise altered from another system. This
checking maintains data integrity.
344 ABCs of z/OS System Programming Volume 3