File control API costs

For read operations, the VSAM I/O cost is not included because the need to access DASD depends on the workload. For the read operation to complete, both the index and data must be accessed. If the index or data are not in a buffer, an I/O operation is required 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:
  • 9.5 for a key-sequenced data set (KSDS)
  • 9.5 for an entry-sequenced data set (ESDS)
  • 8.2 for a relative record data set (RRDS)

READ

KSDS ESDS RRDS Data Table (CMT)
3.0 2.4 2.2 First: 1.5 Subsequent: 1.1

READ UPDATE

Recoverable and nonrecoverable files are included in the READ UPDATE cost:

Table 1. Nonrecoverable files
KSDS ESDS RRDS
3.1 2.3 2.2

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.

KSDS ESDS RRDS
5.5 4.3 4.2

REWRITE

Recoverable and nonrecoverable files are included in the REWRITE cost. Every REWRITE has a data VSAM I/O associated with it.

Table 2. Nonrecoverable files
KSDS ESDS RRDS
10.2 10.1 10.1

A REWRITE of a recoverable file requires that the log buffer that containing the before image is 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 sync pointing if recoverable resources were updated. See Sync pointing.

KSDS ESDS RRDS
10.4 10.3 10.3

WRITE

The cost for WRITE includes nonrecoverable files and recoverable files. Every WRITE has a data VSAM I/O associated with it. The index needs to be written only when a control area split occurs.

Table 3. Nonrecoverable files
KSDS ESDS RRDS
12.9 11.1 10.9

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, the data, or both are not in the buffer. Each WRITE to a recoverable file requires that the log buffer containing the data image has been written out before the VSAM I/O takes place.

At the end of the transaction, there are additional costs involved in sync pointing if recoverable resources were updated. See Sync pointing.

Table 4. Recoverable files
KSDS ESDS RRDS
14.9 13.1 12.9

DELETE

You cannot delete from an ESDS record file.

Table 5. Nonrecoverable files
KSDS RRDS
12.5 11.5

At the end of the transaction, additional costs are involved in sync pointing if recoverable resources were updated. See Sync pointing.

Table 6. Recoverable files
KSDS RRDS
14.5 13.5

Browsing

STARTBR READNEXT READPREV RESETBR ENDBR
3.1 1.5 1.6 2.6 1.4

UNLOCK

The path length for EXEC CICS UNLOCK is 0.7.