This section contains Diagnosis, Modification, or Tuning Information.
If all the data and index records of a file are completely contained in an LSR pool, defining the file as a CICS®-maintained data table does not reduce DASD I/O activity. There is, however, considerable potential for reduction in CPU consumption. Also, you might be able to reduce the number of buffers in the LSR pool.
If the file is not completely contained in an LSR pool, using a CICS-maintained data table could result in reductions in both DASD I/O activity and CPU consumption.
The saving of CPU consumption for a CICS-maintained data table, compared with a VSAM KSDS resident in a local shared resource (LSR) pool, depends on the application usage.
After the loading of a user-maintained data table, DASD I/O activity is eliminated from all data table operations, so the saving of CPU consumption compared with a VSAM KSDS resident in an LSR pool is considerable.
Shared data tables provide efficient use of data in memory. This means that considerable performance benefits are achieved at the cost of some additional use of storage.
This overview of the use of storage assumes that you understand the distinction between various types of storage, such as real and virtual storage, and address space and data space storage.
SDT uses virtual storage as follows:
If many records are increased in length after loading, or new records are added randomly throughout a large part of the file, the amount of storage will be increased, possibly up to twice the original size.
There is one entry for each record in the table, plus one entry for each gap in the key sequence (where one or more records have been omitted from a CICS-maintained data table). The size of each entry is the keylength + 9 bytes, rounded up to the next multiple of 8 bytes.
The size of this area depends on the distribution and format of the key values as well as the actual number of records, as indicated in Table 1.
Key distribution | Key format | Bytes per record |
---|---|---|
Dense (all keys are consecutive) |
binary decimal alphabetic |
5.1 8.5 19 |
Sparse (no keys are consecutive) |
decimal alphabetic |
44 51 |
Worst possible case | - | 76 |
Converting a file into a shared data table could lead to an increased use of real storage, but the use of real storage for VSAM LSR buffers might be reduced if few updates are made. Also, an application that currently achieves high performance by replicating read-only tables in each CICS region might be able to make large storage savings by sharing a single copy of each table.
A data table maintains its own storage within the data space.
It maintains free space, and reuses it when appropriate. When a 128KB frame
is emptied and freed, it goes back on a free chain for that particular data
table. Other data tables within the data space cannot reuse that frame; only
the data table to which it was originally allocated can reuse it. For example,
if a data table grows to 1G, then all the records are deleted from that data
table, the data table still owns 1G of data space storage. No other data table
can use that storage until the owning data table is deleted or destroyed.
The most efficient utilization of data space storage happens when
the number of records written to each data table is approximately equal to
the number of deletes from that data table, so that the size of the data table
remains a constant. In this situation, the amount of storage allocated to
data tables is close to the amount of storage actually in use.
When new data table applications are introduced, it can be helpful
to monitor the storage allocated and storage in use for each data table, to
ensure that you do not reach the 2G limit on the size of the data space. The
readings for storage allocated show the storage owned by each data table,
which will not be given up until the data table is deleted. The readings for
storage in use show how much of the allocated storage is actually in use.
The CICS sample statistics program DFH0STAT provides this information. DFH0STAT
is described in the CICS Performance Guide.