Read VSAM Interview Questions and Answers for Interview PDF
List of VSAM Interview Questions and Answers PDF
What is VSAM?
VSAM stands for Virtual Storage Access Method. It’s a Data Management method used to organize, store, catalog, retrieve and delete the datasets in mainframe system (VSAM and Non-VSAM). This access method makes it easier for an application to execute an I/O operation (moving data between an I/O device and memory). In fact VSAM is the first access method designed for Virtual Storage Environment.
VSAM supports fixed as well as variable length records. These records are placed in blocks of Control Intervals and these control intervals are further placed as Control Area (CA consists of multiple CIs).
What are the advantages of VSAM?
Accessing the data is faster and easier.
VSAM supports more dataset types.
Supports variety of I/O Techniques.
Records can be accessed sequentially or randomly.
What are the disadvantages of VSAM?
VSAM Datasets require more storage space compared to other type of datasets due to Control information(CI) present in them.
VSAM can’t be stored in TAPE Volume i.e., VSAM Stored only in DASD.
VSAM is not a database like DB2, IMS DB & IDMS.
What are the different types of VSAM files available?
ESDS: Entry Sequence Data Set
KSDS: Key Sequence Data Set
RRDS: Relative Data Set
What is purpose of VSAM?
VSAM acts as an interface between Application program & Operating System.
What is the maximum record length for the VSAM dataset?
Max size of a record in VSAM is cylinder = 2880 bytes approx.
What is an alternate index and path?
An alternate index is an another way of accessing key sequenced data record stored in a base cluster and path is the linkage which connect alternate index to its base cluster.
Is a delete operation possible in an ESDS? B, is rewrite operation possible in ESDS?
No delete operation is not possible in VSAM ESDS.B. yes rewrite operation is possible in an ESDS.
What is the difference between LDS and ESDS?
There two datasets are VSAM datasets. ESDS maintains control information. But LDS does not maintains the control information.
How to delete a member using JCL?
Using IDCAMS a member can be deleted. DELETE ‘XXX.YYY (memeber)
What is IDCAMS? and what is the purpose of it?
IDCAMS is an access method services utility used for creating, deleting, altering VSAM files and coping sequential file to a VSAM file, etc.
What are the optional parameters to the input dataset While loading the empty cluster with the data records?
2) TOADDRESS(address) where ‘address’ specifies the RBA value of the key of the input record.
3) FROMNUMBER (rrn)
4)TONUMBER (rrn) where ‘rrn’ specifies the relative record number of the RRDS record
5) FROMKEY (key)
6) TOKEY (key) where ‘key’ specifies the key of the input record
7) SKIP (number)
8) COUNT (number) where ‘number’ specifies the number of records to skip or copy Ex: REPRO INFILE(DD1) OUTFILE(DD2) SKIP(9000) COUNT(700)-Skips the first 9000 records and begin copying at 9001 and copies 700 records from DD1 to DD2.
What does the KEYRANGES parameter in Define Cluster commends do?
It divides a large dataset into several volumes according to the Key ranges specified. e.g. KEYRANGES ((0000001 2999999) (3000000 5999999)). if the activity on the key ranges are evenly distributed, concurrent access is possible, which is a performance improvement.
Describe SHAREOPTIONS parameter (SHR) in Define Cluster command?
It define the cross-region and cross-system sharing capabilities of the dataset. Syntax is SHR (Crvalue, CSvalue) value 1 means multiple read OR single write (read integrity ) 2 means multiple read AND single write ( Write integrity ) 3 means Multiple read AND multiple write 4 is same as 3, which refreshes the buffer with every random access default is SHR(1 3).
What is RECOVERY and SPEED parameters in DEFINE CLUSTER command?
RECOVERY (default ) and SPEED are mutually exclusive. Recovery pre-formats the control areas during the initial dataset load, if the job fails, you can restart but you must have a recovery routine already written to restart the job. SPEED does not pre-format the CAs. It is recommended that you specify SPEED to speed up your initial data load.
Is it slower if you access a record through ALT INDEX as compared to Primary INDEX?
Yes. Why Because the alternate key would first locate the primary key, which in turn locates the actual record. Needs twice the number of I/Os.
How many Alternate Indexes you can have on a dataset?
255 – but you must be a nut to have so many ALT Indexes on a dataset!
On which dataset You can have ALT INDEX?
Only on KSDS and ESDS – not RROS
What is the meaning of VSAM RETURN-CODE 28?
Out of space condition is raised.
How do you fix the problem associated with VSAM out of space condition?
Define new VSAM dataset allocated with more space.
Use IDCAMS to REPRO the old VSAM file to new VSAM dataset.
Use IDCAMS to ALTER / rename the old VSAM dataset or se IDCAMS to DELETE the old VSAM datset.
Use IDCAMS to ALTER / rename the new VSAM dataset to the name of the original VSAM dataset.
During processing of a VSAM file, some system error occurs and it is subsequently unusable What do you do?
Suppose a generation of GDG gets created in a particular step of a proc. How would you refer the current generation in a subsequent step? What would be the disposition of this generation now?
Relative generation numbers are updated only at the end of the job, not at the end of a step. To allocate a new generation, we would be using (+1) with a DISP of ( NEW, CATLG, DELETE ). To refer to this in a subsequent step in the same job, we would again use (+1) but with a DISP of SHR or OLD.
How are different versions of GDG named?
base-file-name. GnnnnnnV00 where nnnn=generation number (upto 255). nnnn will be 0000 for the 1st generation.
How do you define a GDG?
Use the DEFINE GENERATIONDATAGROUP command. In the same IDCAMS step, another dataset must be defined whose DCB parameters are used when new generations of the GDG are created. This dataset is known as the model dataset. The DS name of this model dataset must be the same as that of the GDG, so use a disp of keep rather than catlg and also specify space = (trk,0)
What is the difference between sequential files and ESDS files?
Sequential (QSAM) files can be created on tape while ESDS files cannot. Also, you can have ALTINDEX for an ESDS while no such facility exists for QSAM files.
How do you calculate record size of an alternate cluster? Give your values for both unique and nonunique?
UniqueCase: 5 + (alt-key-length + primary – key )
Non unique Case: 5 + (alt-key-length + n * primary – key ) where n = number of duplicate records for the alternate key
How do you load a VSAM data set with records?
Using the REPRO command.
Do all versions of the GDG have to be of the same record length?
No, the DCB of the model dataset can be overridden when you allocate new versions.
Suppose 3 generations of a GDG exist. How would you reference the 1st generation in the JCL?
Use GDG name(-2).
What more info you should give in the DD statement while defining the next generation of a GDG?
Give (+1) as the generation number, give (new, catlg) for disp, give space parameter, can give the DCB parameter if you want to override the DCB of the model dataset.
Assuming that the DEFINE JCL is not available, how do you get info about a VSAM files organisation?
Use the LISTCAT command.
Using Alternate Indexes in CICS program?
FCT entries must be created for both base cluster and the path. To read using the alternate index, use the DD name of the path in CICS file control commands.
Using Alternate Indexes in Batch program?
In the JCL, you must have DD statements for the cluster and for the path9s). In the COBOL program, SELECT … ASSIGN TO ddname for base cluster RECORD KEY IS… ALTERNATE RECORD KEY IS …
What is a LDS (Linear Data Set) and what is it used for?
LDS is a VSAM dataset in name only. It has unstructured 4k (4096 bytes ) fixed size CT’s which do not contain control fields and therefore from VSAM’s standpoint they do not contain any logical records. There is no free space, and no access from Cobol. Can be accessed by DB2 and IMS fast path datasets. LDS is essentially a table of data maintained on disk. The ‘table entries’ must be created via a user program and can only be logically accessed via a user program. When passed, the entire LDS must be mapped into storage, and then data is accessed via base and displacement type processing.
What is the meaning of the DEFINE MODEL parameter?
It specifies whether Daniela Pestova or Yamila – oops! Wrong models! The MODEL parameter allows you to model your cluster by modeling it after an existing cluster.
What are the three levels of definition for the VSAM DEFINE?
They are DEFINE CLUSTER, DATA and INDEX.
What is the utility program closely associated with VSAM?
IDCAMS, the access method services utility.
What is the VSAM-code field?
It is a COBOL II enchancement to VSAM batch processing expanding the FILE STATUS field. It is defined in WORKING-STORAGE as a six byte group item with three two byte elements, the normal return code, the function code and the feedback code.
What is the meaning of dynamic processing?
It’s rarely used. It means one program uses both sequential and random processing for a VSAM KSDS file.
Under IDCAMS, multiple functions can be executed, each of which returns a condition code. What will be the condition code returned to the operating system?
The maximum condition code generated is returned as the condition code of the IDCAMS step.
Would you specify FREESPACE CE for an ESDS?
No. Because you cannot insert records in an ESDS, also when you rewrite a record, it must be of the same length. Thus putting any value for FREESPACE does not make any sense.
What is the meaning of each of the values in SHAREOPTS (2 3)?
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).
How do you define an ALTINDX? How do you use ALTINDXs in batch, CICS programs?
DEFINE ALTERNATEINDEX. Important parameters are RELATE where you specify the base cluster name, KEYS, RECORDSIZE, SHAREOPTION, UNIQUEKEY (or NONUNIQUEKEY), DATA (DS NAME for the data component), INDEX(DS name for the index component). Then DEFINE PATH. Important parameters are NAME (DS name for the path). PATHENTRY (DS name of the alternate index name). UPDATE (or NOUPDATE) which specifies whether an ALT index is updated when a update to the vase cluster takes place. Then BLDINDEX. Paramters are INDATASET (DS name of base cluster), OUTDATASET (DS name of AIX).
Name some common VSAM error conditions and codes?
They are end of file (10), duplicate key (22), record not found (23), VSAM logic error (90), open problem (92) and space problem (93).
What is a VSAM slot?
A relative record dataset (RRDS) consists of a specified number of areas called slots. Each slot is identified by a relative record number (RRN) which indicates its relative position in the file.
There are at least seven IDCAMS commands name and explain each of them?
ALTER modifies information for a catalog, alternate index, cluster or path. BLDINDEX builds the alternate index, of course. DEFINE is used for ALTERNATEINDEX, CLUSTER or PATH. DELETE removes the catalog entry for a catalog, cluster, alternate index or path. LISTCAT lists information about the dataset. PRINT prints the dataset contents. REPRO copies records from one file to another.
What is the significance of the SHAREOPTIONS parameter?
It specifies how the file may be shared between jobs and between batch and CICS environments.
What is File Status in VSAM?
The File STATUS clause of the FILE-CONTROL paragraph allows for each file to be associated with a file status key (i.e., the 2-character data item specified in the FILE STATUS clause). If the FILE STATUS clause is specified for a given file, a value indicating the status of each I/O operation against that file is placed in the associated file status key. This value is stored in the file status key as soon as the I/O operation is completed (and before execution of any EXCEPTION/ERROR declarative or INVALIDKEY/AT END phrase associated with the I/O request).
Note: This element may behave differently when the CMPR2 compiler option is used. The file status key is divided into two status keys: the first character is known as file status key1; the second character is file status key 2.
If you wish to use the REWRITE command haw must the VSAM file be opened?
It must be opened as I/O.
What is the COBOL RECORD KEY clause?
The RECOED KEY in the SELECT clause identifies the files primary key as it will be known to the program.
In the COBOL SELECT statement what is the ORGANIZATION for a KSDS?
The ORGANIZATION is INDEXED.
What is the base cluster?
The base cluster consists of the data component and the index component for the primary index of a KSDS.
What is free space?
Free space is reserved within the data component of a KSDS to accommodate inserting new records.
What is a path?
A path is a file that allows you to access a file by alternate index – the path provides an association between the AIX and the base cluster.
What is a CI, control interval?
A control interval is the unit of information that VSAM transfers between virtual and auxilliary storage.
What is a CA, control area?
A group of control intervals makes up a control area.
What is the index set?
This is the other part of the index. It has multiple levels with pointers that ultimately reach to the sequence set.
What is the catalog?
The catalog contains the name of all datasets. VSAM and non-VSAM. It is used to access these datasets.
What is the purpose of the FILE STATUS clause in the SELECT statement?
The FILE STATUS field identifies the field that VSAM uses to provide information about each I/O operation for the file.
In the COBOL SELECT statement for a KSDS what are the three possibilities for ACCESS?
ACCESS can be SEQUENTIAL, RANDOM or DYNAMIC.
Do primary key values have to be unique? Do alternate key values have to be unique?
Primary key values must be unique; alternate key values need not be.
What is a VSAM split?
If there isn’t enough space in the control interval VSAM performs a control interval split by moving some records to the free control intervals. If there isn’t a free control interval VSAM performs a control area split by allocating a new control area and moving half of the control intervals to it.
What is the upgrade set?
The upgrade set is the list of all AIXes that VSAM must maintain for a specific base cluster, so that when data in the base cluster is updated, the AIX files are also updated.
What is an alternate index?
An AIX is a file that allows access to a VSAM dataset by a key other than the primary one.
What is a cluster?
A cluster is the combination of the index, sequence set and data portions of the dataset. The operating system gives program access to the cluster, ie. to all parts of the dataset simultaneously.
What is a sequence set?
This is the part of the index that points to the CA and CI of the record being accessed.
What are the distinctive features of a KSDA, key sequenced dataset?
The index and the distributed free space.
How are records stored in an ESDS, entry sequenced dataset?
They are stored without respect to the contents of the records and in the order in which they are included in the file.
What are the types of VSAM datasets?
Entry sequenced dataset (ESDS), key sequenced datasets (KSDS) and relative record dataset (RRDS).