An Improved Index Sequential Access Method Using Hashed Overflow
The Index Sequential Access Method (ISAM) is
one of the most important file management systems 
used with moveable head disk devices.  This study investigates
the use of an unconventional method of 
treating overflow records.  The method is to use hashing
techniques to allocate space for such records. 
 If certain conditions are satisfied, this is superior
to the conventional ISAM method of chaining the 
overflow records via linked list techniques.  These conditions are:
long overflow chains with significant 
overflow; lack of tight disk space constraints; record
keys which are small compared to the total record 
size; and significant use of the file in the index as opposed
to the sequential mode.  Using hashed overflow, 
the time to locate a record is dependent not on the total
volume of overflow records as in conventional 
ISAM, but on the percentage use of space dedicated to overflow records.
CACM May, 1972
Mullin, J. K.
