This information applies to non-SMS-managed volumes. With SMS-managed volumes, SMS selects the volumes. When a key-sequenced data set or variable-length RRDS is defined, the entire index or the high-level index set alone can be placed on a volume separate from the data, either on the same or on a different type of device.

Using different volumes lets VSAM gain access to an index and to data at the same time. Also, the smaller amount of space required for an index makes it economical to use a faster storage device for it.

A performance improvement due to separate volumes generally requires asynchronous processing or multitasking with multiple strings.