Indexed files are more complex than sequential organization. These files provide both sequential and direct access to the records. Indexed files can exist only in on disk. You can copy its contents to tape, but you can't directly access the data on tape. These are the modern way of utilizing the power of disk storage. There are two types of indexed files: ISAM and VSAM. ISAM or Indexed Sequential Access Method is a trademark of IBM for its first and now obsolete way to provide indexed files. VSAM stands for Virtual Storage Access Method and is IBM's current way of implementing indexed files. VSAM's internal organization is complex and as shown in the figure the data is stored in the data component and the information about the data is stored in the index component, which is stored separately. You need a special utility called IDCAMS to create and manage VSAM datasets. The following figure shows a VSAM data set.

To work effectively with MVS you need to understand how it keeps track of datasets. MVS has two more levels of information above the PDS directory. A Mainframe might have hundreds of disk drives attached to it. The disk 'Volume' is the assembly of the magnetic platters inside the disk drives. Datasets (PS, PDS, VSAM) each exist as recorded information in one or more disk volumes. Each disk volume has its own identifies and contains millions of bytes of storage capacity. This identifier is called the volume serial number and could be a combination of letters and numbers and six characters in length. Recorded in each disk volume is a Volume Table of Contents (VTOC). This is a special dataset managed by MVS to keep track of all the datasets on the volume. Records in the VTOC point to datasets in the disk and keep important information about each dataset. MVS maintains a critical dataset on a special disk volume. This is the MVS system catalog. It contains information about where or which disk volume each dataset resides. MVS keeps information about a dataset by keeping its name and location in the catalog.

VSAM Data Set

VSAM Data Set

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