Types of DASD Sequential Data Sets

There are three types of sequential data sets that reside on DASD. You can choose the type by coding DSNTYPE on the DD statement or the dynamic allocation equivalent, by using a data class that has DSNTYPE, or by using the LIKE parameter. These types of data sets are distinct from members of a PDS or PDSE, or from UNIX files. Table 1 summarizes the types of sequential data sets, with their characteristics and advantages.

Table 1. Types of sequential data sets on DASD – characteristics and advantages
	Basic Format	Large Format	Extended Format
DSNTYPE	BASIC	LARGE	EXTREQ (required) or EXTPREF (preferred)
Maximum number of tracks on a volume	65␠535	16␠777␠215	As many as on the largest DASD volume
Maximum number of extents on a volume	16	16	123
Advantages	Maximum compatibility. Allows EXCP and BDAM, SMS and non-SMS.	Can be much larger than basic format. Allows EXCP, SMS and non-SMS.	Can be much larger than basic format. Can be striped, compressed format or any combination.