sec isam in-memory table of cylinder keys cylinder cylinder index cylinder cylinder index records records cylinder cylinder overflow overflow system overflow figure illustration of the isam indexing system in the index inverted lists reduce this problembut they are only suitable for secondary key indices with many fewer secondary key values than records the linear index would perform well as primary key index if it could somehow be broken into pieces such that individual updates affect only part of the index this concept will be pursued throughout the rest of this eventually culminating in the -treethe most widely used indexing method today but firstwe begin by studying isaman early attempt to solve the problem of large databases requiring frequent update its weaknesses help to illustrate why the -tree works so well before the invention of effective tree indexing schemesa variety of disk-based indexing methods were in use all were rather cumbersomelargely because no adequate method for handling updates was known typicallyupdates would cause the index to degrade in performance isam is one example of such an index and was widely used by ibm prior to adoption of the -tree isam is based on modified form of the linear indexas illustrated by figure records are stored in sorted order by primary key the disk file is divided among number of cylinders on disk each cylinder holds section of the list in sorted order initiallyeach cylinder is not filled to capacityand the extra space is set aside in the cylinder overflow in memory is table listing the lowest key value stored in each cylinder of the file each cylinder contains table listing the lowest recall from section that cylinder is all of the tracks readable from particular placement of the heads on the multiple platters of disk drive