Mainframe VSAM Interview Questions and Answers

10/24/2009 No Comment

VSAM Interview Questions and Answers and Solutions.

Can you tell me whether in ESDS, do we have the facility of accessing the records randomly?
Answer : Yes, Random access of records is possible. However, records cannot be deleted. We have to do it by referring the RBA (relative byte address).

Explain in brief what is the difference between ESDS and KSDS in VSAM? 
Answer : ESDS doesn’t have imbedded free space.The KEYS parameter has no meaning in the context of ESDS. ESDS has no index component. An additional parameter, NONINDEXED is used to tell AMS that an SDS is being allocated.

Why is the space is kept in data component of the KSDS Dataset in VSAM?
Answer : Free space specified during the allocation of the KSDS is left at regular intervals during the initial load of the data set. This space helps in keeping the data component in physical sequence in spite of Random insertions.

Can you explain how is LDS different from ESDS in VSAM?

Answer : An LDS is a data set; very similar to an ESDS without the control information .It has no records and used for fast random access. It takes advantages of the computer’s very fast paging hardware for retrieval and storage and the bytes are implicitly divided into 4K blocks or pages, and the paging hardware reads and writes blocks from disk. There is no free space, unused space, control field in LDS.

What do you mean by CI, Control Interval in VSAM?
Answer : A Control Interval is the unit of information that VSAM transfers between virtual and auxiliary storage.

What do you mean by Control Interval Split?
Answer : A new record stored in same CI, only if there is enough space. If not VSAM locates the free CI within the same CA, moves approximately half of the records to next CI and stores the new record at the correct position. This is called Control Interval Split. Whenever CI split occurs the sequence set is updated.

What do you mean by a CA, control area? 
Answer : A group of Control Intervals makes up a control area.

What do you mean a sequence set?

Answer : Sequence Set is the part of the index that points to the CA and CI of the record being accessed.

Explain what is a cluster?

Answer : A VSAM cluster is the combination of the index, sequence set and data portions of the Dataset. The operating system gives program access to the cluster, i.e. to all parts of the Dataset simultaneously.

What is an alternate index in VSAM. Explain?
Answer : An AIX is a file that allows access to a VSAM Dataset by a Key other than the primary one. The Alternate Key-pointer pair records are stored in an index cluster. This index cluster is known as Alternate index. The alternate Key-pointer pair records are loaded into the alternate index cluster by the command BLDINDEX.AIX can be built over a KSDS and ESDS, but not over an RRDS.

What do you mean by a index set in VSAM?
Answer : This is the other part of the index. It has multiple levels with pointers that ultimately reach to the sequence set.

What do you man by a Catalog in VSAM?

Answer : The catalog contains the names of all Datasets, VSAM and non-VSAM. It is used to access these Datasets.

Can you tell me what could be the maximum number of the AIXs per base cluster?

Answer : There can be a maximum of 253 AIXs per base cluster. It is not recommended to have more than 5 AIXs per base cluster because of additional overhead during updates and retrievals.

Can AIX be defined over an ESDS, which does not have any Key?
Answer : Although an ESDS does not have a primary Key, an AIX can be defined over an ESDS. In the case of an ESDS it is the relative byte address of the corresponding record in the base cluster that is stored.

Can we access records in VSAM only by a single Key, i.e. primary Key?

Answer : Yes, it is also possible to access the records in a sequence other than that of the primary Key. Such Keys are called alternate Keys and they can be non-unique.

For Instance, in a pay-roll system where employee number is the unique primary Key and the Employee name as alternate Key.

What is the 'verify' command in the VSAM files?

Answer : This  is a command that is used to close those files that are kept open after the abnormal termination of the file. It also brings the index component in sync with the data components and updates the catalog in VSAM.

Explain what do you mean by a path?

Answer : A path is basically a file that allows you to access a file by an alternate index - the path provides an association between the AIX and the base cluster.
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