This section contains the relative costs of VSAM file control accesses. For read operations the VSAM I/O cost is not included because the necessity to access DASD is workload dependent. For the read operation to complete both the index and data must be accessed. If neither index or data is in a buffer, an I/O must be done for each level of index and one for the data. The relative number of instructions, in 1K instruction counts, for the I/O for each file type is as follows:
KSDS | ESDS | RRDS | Data Table (CMT) |
---|---|---|---|
3.0 | 2.4 | 2.2 | First: 1.5 Subsequent:1.1 |
Recoverable and non-recoverable files are included in the READ UPDATE cost:
KSDS | ESDS | RRDS |
---|---|---|
3.1 | 2.3 | 2.2 |
KSDS | ESDS | RRDS |
---|---|---|
5.5 | 4.3 | 4.2 |
Notes:
A recoverable READ UPDATE puts the 'before image' into the log buffer which, if not subsequently written to primary storage, is written out before the REWRITE is completed. |
Recoverable and non-recoverable files are included in the REWRITE cost.
Every REWRITE has a data VSAM I/O associated with it.
KSDS | ESDS | RRDS |
---|---|---|
10.2 | 10.1 | 10.1 |
KSDS | ESDS | RRDS |
---|---|---|
10.4 | 10.3 | 10.3 |
Notes:
A REWRITE of a recoverable file requires that the log buffer containing the before image has been written out. If the buffer has not already been written out since the READ UPDATE, the cost of writing the log buffer is incurred. When the before image has been hardened the VSAM I/O takes place. At the end of the transaction, there are additional costs involved in syncpointing if recoverable resources have been updated. See Syncpointing. |
The cost for WRITE includes nonrecoverable files and recoverable files.
Every WRITE has a data VSAM I/O associated with it. The index will need to be written only when a control area split occurs.
KSDS | ESDS | RRDS |
---|---|---|
12.9 | 11.1 | 10.9 |
KSDS | ESDS | RRDS |
---|---|---|
14.9 | 13.1 | 12.9 |
Notes:
Every WRITE has a hidden READ associated with it to ensure that the record is not already present in the file. This under the cover READ could incur the cost of I/Os if the index and/or data are not in the buffer. Each WRITE to a recoverable file will require that the Log Buffer containing the data image has been written out before doing the VSAM I/O. At the end of the transaction, there are additional costs involved in syncpointing if recoverable resources have been updated. See Syncpointing. |
You cannot delete from an ESDS record file.
KSDS | RRDS |
---|---|
12.5 | 11.5 |
KSDS | RRDS |
---|---|
14.5 | 13.5 |
Notes:
At the end of the transaction, additional costs are involved in syncpointing if recoverable resources have been updated. See Syncpointing. |
STARTBR | READNEXT | READPREV | RESETBR | ENDBR |
---|---|---|---|---|
3.1 | 1.5 | 1.6 | 2.6 | 1.4 |
The pathlength for EXEC CICS® UNLOCK is 0.7.
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