MECH 2301 Lecture Notes - Lecture 25: Ibm Mainframe, Access Method, Sequential Access
22 May 2018
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MECH 2301 Lecture 25 Notes
Introduction
Physical file storage
• Indexes provide an additional means for accessing specific records in a file.
• A file may have multiple indexes, each representing a different way of viewing the data.
• A telephone list could be indexed by address, by name, and by phone number, for
example.
• The index provides pointers that can immediately locate a particular logical record.
• Furthermore, an index is often small enough that it can be kept in memory for even
faster access.
• Indexes are generally used in combination with sequential and random access methods
to provide more powerful access methods.
• Simple systems normally provide sequential and random access at the file manager level
and rely on application programs to create more complex methods of access.
• Large systems provide additional access methods.
• The most common of these is the indexed sequential access method (ISAM).
• ISAM files are kept sorted in order of a key field.
• One or more additional index files are used to determine the block that contains the
desired record for random access.
• The IBM mainframe operating system z/OS provides six different access methods, and
one of these, VSAM, is further divided into three different sub methods.
• All these additional methods are built upon either random or sequential access or a mix
of the two and use index files to expand their capability.
• The file manager allocates storage based on the type of I/O device, the file access
method to be used for the particular file, and the particular design of the file manager.
• There are three primary file storage methods used for random access devices, such as
disks.
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