VSAM Interview Questions asked in MNCs

9/17/2009 No Comment

IBM VSAM Interview Questions and Answers.

Name the different types of VSAM files available?
Answer : The different types of VSAM files available.
ESDS: Entry Sequence Data Set
RRDS: Relative Data Set
KSDS: Key Sequence Data Set

What do you mean by IDCAMS ?
Answer : IDCAMS is nothing but the Access Method Services program. You can run the IDCAMS program and supply AMS commands thru SYSIN. Some examples of AMS commands are DELETE, REPRO, DEFINE etc.

Can you run AMS commands from the TSO prompt ?

Answer : Yes, you can run AMS commands from TSO prompt.

Under IDCAMS , there are multiple functions that can be executed, each of which returns a condition code. What will be the condition code returned to the operating system ? 
Answer : The maximum condition code generated is returned as the condition code of the IDCAMS step.

Explain in detail what is Control Interval and Control Area in VSAM?
Answer : Control Interval is analogous to a physical block for QSAM files. It is the unit of input/output. It should be between 512 bytes to 32 K. A larger control interval increases performance for sequential processing while the reverse is true when you use it for random access. 
Under CICS, when a record is locked, the entire Control Interval gets locked.

Whereas Control area is a group of control intervals. CA is used during allocation. CA size is calculated based on the allocation type (cyl, tracks or records) and can be max of 1 cylinder.

Explain how do you decide on optimum values for Control Interval, FREESPACE in VSAM?

Answer : Control Interval size should be based on record length, type of processing. Usually CI is 4K. If record length is larger(>1K), chose 6K or 8K.

FREESPACE should be large if more number of insertions are envisaged. Usual values are (20 20) when heavy updates are expected. CI size can be calculated.

Can you explain the meaning of each of the values in SHAREOPTS(2 3)?
Answer : Value of 2 for cross region means that the file can be processed simultaneously by multiple users provided only one of them is an updater. Value of 3 for cross system means that any number of jobs can process the file for input or output (VSAM does nothing to ensure integrity).

Is there a need to specify FREESPACE for an ESDS?

Answer : No, there is no need to specify FREESPACE. As, you cannot insert records in an ESDS, also when you rewrite a record, it must be of the same length thereby putting any value for free space does not make any sense.

What do you mean by SHAREOPTS in VSAM?

Answer : SHAREOPTS is just a parameter in the DEFINE which specifies how an object can be shared among users. It is coded as SHAREOPTS(a b), where a is the cross region share option i.e how two or more jobs on a single system can share the file, while b is the cross system share option i.e how two or more jobs on different MVSes can share the file. Usual value for SHAREOPTS is (2 3).
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