VSAM Interview Questions | Freshers & Experienced

VSAM stands for Virtual Storage Access Method. VSAM is a file storage access method used in MVS, ZOS and OS/390 operating systems. It was introduced by IBM in the 1970's. It is a high-performance access method used to organize data in the form of files in Mainframes.

There are 4 types of VSAM files. They are

• KSDS (Key Sequential DataSet)
• ESDS (Entry Sequential DataSet)
• RRDS (Relative Record DataSet)
• LDS (Linear DataSet)

ESDS is known as Entry Sequenced Data Set. An entry-sequenced data set behaves like a sequential file organization with some more features included. We can access the records directly and for safety purposes, we can use passwords also. We must code NONINDEXED inside the DEFINE CLUSTER command for ESDS datasets.

SHAREOPTS is a parameter in the DEFINE and specifies how an object can be shared among users. It is coded as SHAREOPTS(a b), where a is the cross-region share option ie how two or more jobs on a single system can share the file, while b is the cross-system share option ie how two or more jobs on different MVSes can share the file.

Control Intervals are the smallest unit of transfer between a disk and the operating system. Whenever a record is retrieved directly from the storage, the entire Control Interval containing the record is read into VSAM Input-Output buffer.

The average record length in KSDS must be less than or equal to the maximum record length. The Maximum Record Length is 2046.

The differences between VSAM and Non-VSAM files are as follows:

VSAM -

• VSAM files require the utility to create, edit and delete the file.
• Only works with data stored on a disk and can't handle data stored on other devices like tapes.
•  The information of VSAM files is stored in Catalogs.

Non-VSAM -

• Non-VSAM files can be created, edited, and deleted using the ISPF facility.
• Files can store on both disks and tapes.
• The information about the Non-VSAM files is stored as dataset labels in the VTOC.

ALTER command is used to modify VSAM file attributes. We can change the attributes of VSAM file which we have mentioned in VSAM Cluster definition. Following is the syntax to change the attributes −

ALTER  file-cluster-name [password] 
   [ADDVOLUMES(volume-serial)] 
   [BUFFERSPACE(size)] 
   [EMPTY / NOEMPTY] 
   [ERASE / NOERASE] 
   [FREESPACE(CI-percentage CA-percentage)] 
   [KEYS(length offset)] 
   [NEWNAME(new-name)] 
   [RECORDSIZE(average maximum)] 
   [REMOVEVOLUMES(volume-serial)] 
   [SCRATCH / NOSCRATCH] 
   [TO(date) / FOR(days)] 
   [UPGRADE / NOUPGRADE] 
   [CATALOG(catalog-name [password]]

The different types of record management in VSAM are as follows:

• Entry Sequenced Data Set (ESDS)
• Key Sequenced Data Set (KSDS)
• Linear Data Set (LDS)
• Relative Record Data Set (RRDS)

The records are stored in an ESDS in the order in which they were first loaded into the data set, such that the new records added to an ESDS always go after the last record in the data set. Once a record has been stored in an ESDS, its RBA remains constant.

In VSAM, the Freespace specifies the percentage of free space to reserve for the control intervals or CI and control areas or CA of the data component, such that the default value of this parameter is zero percentage.

You can access the last record from the KSDS file in VSAM by moving the highvalues to the key of the VSAM file and the issue Readnext record command. Then give Readprev command. This will read the last record. In VSAM, there is one command HURBA, Using that we can read the last record, If you want to know more about going to IBM RED BOOKS.

In VSAM, RDW stands for Record Descriptor Word that is a 4-byte field describing the record. The first 2 bytes contain the length (LL) of the logical record (including the 4-byte RDW). The length can be from 4 to 32,760. All bits of the third and fourth bytes must be 0 because other values are used for spanned records.

CISZ stands for Control interval sizes that provides your system programmer with information about the data access and future growth of your VSAM data sets.

A VSAM cluster is a logical definition for a VSAM data set. It is the combination of the index, sequence set and data portions of the dataset. The operating system gives program access to the cluster.

GDG stands for generation data group which is a collection of historically related non-VSAM data sets that are arranged in chronological order. That is, each data set is historically related to the others in the group. Within a GDG, the generations can have like or unlike DCB attributes and data set organizations.

Advantages, Pros, and Cons of VSAM:

VSAM is a quick way of accessing data and it also supports a lot of datasets within one cluster. This advantage is utilized by several people to store, access and maintain data.

Pros:

• VSAM allows sharing of data to different regions/systems
• Data can be protected using a password for different datasets

Cons:

• VSAM consumes a lot of storage space as it has CI component