Hi Gokul -
A Partitioned Dataset is a special form of a Sequential Data-set. Every PDS has a directory(just like Yellow-Pages), to keep track of how its allocated disk space, or data space
, has been divided into parts. Each part(slice) is called a member.
Say for example, you have a PDS with 5 Members, on the disk -
Assuming 8 Blocks/Track,
TRK,BLK 1 2 3 4 5 6 7 8
...... .... .... .... .... .... .... .... ....
0 A A A A B B B B
1 B B C C C D D D
2 D D D E E E E E
3 E E E E E E E E
The PDS directory will contain a list of all the PDS Member-names, and a disk-pointer. The minimum directory block room, you need for each member is 12 Bytes. It has the 8-Byte member-name and a 4-byte binary value called the TTRC(Relative Track Number). The TTRC indicates the relative Track Number and the block where the member starts.
The PDS Directory containing the member-names and pointers
zOS also allows applications like TSO to store "user-data" in PDS Directory entries for each member via Assembler Macros. If you retain the ISPF statistics about each member, as TSO/ISPF normally does, each member takes 42 bytes of directory block space. The last 12 bytes of a directory is specially meant for End-Of-Directory Member.
When you allocate a partitioned data set, you reserve/set-aside space for it on the disk. You generally do this with the TSO/ISPF Function 3.2. At that time, you decide how big its directory will be. 1 Directory block is 256 bytes large. This implies, if you reserve 1 block for the PDS Directory(256 Bytes), with each member occupying 42 bytes of directory block space, the directory would be able to store information about (256 - Last 12 bytes)/42 = 5 Members.
Therefore, a rule of thumb in creating a PDS is to estimate how many members you will want to house in the PDS, and divide it by five to calculate the number of directory blocks.
Its quite possible for a partitioned-data set to have member storage space available, but no ability to house more members, as its directory is filled up. You'll get a system completion code SB14-0C.