Ppt On Vsam

VSAM Virtual Storage Access Method Allows for interactive updates (adds,

changes, and deletes)

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Three Types of VSAM files KSDS Keyed Sequence Data Set RRDS Relative Record Data Set ESDS Entry Sequence Data Set

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KSDS  Allows for users to access records

sequentially or randomly  Includes an index and data components  A CLUSTER consists of both the index and data components together  Index relates a value in a key field to the actual location of the record on disk  Index is only used in random (or dynamic) processing

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VSAM Data Component Storage Concepts  Control Interval (CI)  Fixed amount of storage; Must be multiple of 512  Usually 2048 or 4096 bytes  Data records are stored in a CI • A CI of 4096 bytes can store forty 100-byte records

 Control Areas (CA)  One cylinder in size  Size of a cylinder varies by disk drive  Contains numerous CIs 1-4

Approximate number of records in a CI (CI size - 10) / record length

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Records in a CI

 Calculate the number of records in a CI

formula on previous slide  Determine the percentage of records in a CI to be added or insert in an average CI  Concept of FreeSpace • Space where records can be added “on the fly”

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CI in a CA A fixed number based on CI size and

disk drive

For disk drive with ACD001: • CI of 2048: 315 CI in a CA • CI of 4096: 180 CI in a CA

Determine the percent of the CI to be left open for additions Known as CA freespace 1-7

Freespace in a CI

Is used to add records which belong

in the CI Records in a CI are shifted automatically within the CI to accommodate the inserted record

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CI Splits  If there is no freespace in the CI in

which the record is to be inserted:

 CI split  Half of the records in the CI are moved to a free CI in the same CA  The inserted record is then inserted in the proper CI  These happen routinely and are accomplished quickly

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CA splits  If a CI needs to split and there is no free CI

in the CA: CA split  Half of the CI are moved to a free CA (usually at the end of the file, there is unused space)  Therefore, each of the two CI have 50% free space  The original CI can now split  CA splits have much overhead and should be avoided!!! 1-10

Avoid CA splits  Reorganization of files  Import: Copy the VSAM file to a sequential dataset  Export: Delete and reload the VSAM file from the sequential data set: Resets the freespace as in the define cluster  Allocate adequate freespace  Analyze the primary key

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Primary Key Analysis Is the pattern in the PK Example: The Financial Aid office must keep three years of data on-line. • Previous year: State reporting • Current year: Distributing aid to students • Next year: Granting/guaranteeing aid for

next year

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Primary Key Analysis Key options for the financial aid file  1-digit year + SSN • Little on-line activity on first third of file • Most “adds” are in last third: CI/CS split

likely

SSN + 1 digit year • Activity is spread evenly throughout the file • Recommended

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VSAM Index Component  Every KSDS as an index component for

each primary key AND each foreign key  Base cluster: Primary key index and Data components  Foreign key index is known as the alternate index

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Primary Key Index and Data Two parts of the index: Sequence set Index set

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Primary Key Index and Data  Sequence Set  Lowest level of the index component  Contains information that relates key values to a specific CI  Links the highest PK in a CI to the address of that CI • Stores all the key-address pairs for the Cis in a CA

in one CI of the sequence set • There is a separate CI in the sequence set for each CA in the data

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Primary Key Index and Data  Index Set  Highest level of the index component  Key-address pairs stored in one CI (can be stored in main memory for processing efficiency) • Address Pointer links to address of the appropriate

sequence set • Based on size of data component (number of cylinders or CA needed to store data component), you may need a intermediate index

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Alternate Index and Data Component  Alternate index relates alternate key to

primary key  Then uses the primary key index to locate the data  Can have unique or non-unique AI  Figure 2-4 (unique)  Figure 2-5 (non-unique)

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Alternate Index  Systems analyst determines whether to

update the AI each time the cluster is changed  Can cause much overhead to update all indices especially in the case of a CA split  Other alternative is to re-build the index periodically (every night) 1-19

Relative Record Data Set (RRDS)  Lets the user access each record at random

without the overhead of maintaining an index  Instead each record in a RRDS is numbered, starting with 1 for the first record  RRDS consists of a specified number of areas or slots  Known as the relative record number (RRN)

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RRDS  May need a routine to convert the PK of a

record to a relative record number  Hashing • Most common hashing routine: The

remainder option on the divide  Can cause empty slots. Can waste storage; But we avoid CI/CA splits  Difficult to HASH if PK is non-numeric

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RRDS  Collision  If hashing routine results is same RRN for two different PK  Must set secondary searching technique in case of collisions • Usually linear probing: check the next record up to a maximum number of tries.  Needed to know if record to be added already exists  Needed to determine if record to be retrieved exists without reading entire file

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RRDS  Advantages  No index overhead  Direct relationship between data and location of the data  Permits both random and sequential processing  If good hashing routine with minimal collisions: performance efficiency is excellent

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RRDS  Disadvantages  Storage efficiency  Collisions  Difficulty in determining good hashing technique • Difficulty with alphabetic key  Does NOT support the concept of FK or AI  Not widely used

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ESDS  Entry Sequenced Data Set  The simplest type of VSAM file  Records are stored sequentially at time of

entry

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ESDS Similar to sequential processing Does allow to OPEN EXTEND to add

records to the end of the file

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ESDS

 Author does not recommend it  Says it is restricted to sequential processing  BUT you can build an AI for a FK  BUT as of my current COBOL manuals, COBOL cannot use the AI and processing must be sequential.  This is my current topic for career day questions 1-27