Choosing file organization and access mode

There are several guidelines you can use to determine which file organization and access mode to use in an application.

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Consider the following guidelines when choosing file organization:

If an application accesses records (whether fixed-length or variable-length) only sequentially and does not insert records between existing records, a QSAM or VSAM sequential file is the simplest type.
If you are developing an application for z/OS® UNIX file system that sequentially accesses records that contain only printable characters and certain control characters, line-sequential files work best.
If an application requires both sequential and random access (whether records are fixed length or variable length), a VSAM indexed file is the most flexible type.
If an application inserts and deletes records randomly, a relative file works well.

Consider the following guidelines when choosing access mode:

If a large percentage of a file is referenced or updated in an application, sequential access is faster than random or dynamic access.
If a small percentage of records is processed during each run of an application, use random or dynamic access.

Table 1. **Summary of file organizations, access modes, and record formats of COBOL files**
File organization	Sequential access	Random access	Dynamic access	Fixed length	Variable length
QSAM (physical sequential)	X			X	X
Line sequential	X			X¹	X
VSAM sequential (ESDS)	X			X	X
VSAM indexed (KSDS)	X	X	X	X	X
VSAM relative (RRDS)	X	X	X	X	X
The data itself is in variable format but can be read into and written from COBOL fixed-length records.