Calculating Space

If you want to estimate the space requirements yourself, you need to answer the following questions to estimate your space requirements accurately and use the space efficiently.

What is the average size of the members to be stored on your direct access volume?
How many members will fit on the volume?
Will you need directory entries for the member names only, or will aliases be used? If so, how many?
Will members be added or replaced frequently?

You can calculate the block size yourself and specify it in the BLKSIZE parameter of the DCB or DCBE. For example, if the average record length is close to or less than the track length, or if the track length exceeds 32␠760 bytes the most efficient use of the direct access storage space can be made with a block size of one-third or one-half the track length.

For a 3380 DASD, you might then ask for either 75 tracks, or 5 cylinders, thus permitting for 3␠480␠,000 bytes of data. Assuming the allocation size of 3␠480␠000 bytes and an average length of 70␠000 bytes for each member, you need space for at least 50 directory entries. If each member also has an average of three aliases, space for an additional 150 directory entries is required.

Each member in a data set and each alias need one directory entry apiece. If you expect to have 10 members (10 directory entries) and an average of 3 aliases for each member (30 directory entries), allocate space for at least 40 directory entries.

Space for the directory is expressed in 256-byte blocks. Each block contains from 3 to 21 entries, depending on the length of the user data field. If you expect 200 directory entries, request at least 10 blocks. Any unused space on the last track of the directory is wasted unless there is enough space left to contain a block of the first member.

Any of the following space specifications would allocate approximately the same amount of space for a 3380 DASD. Ten blocks have been allocated for the directory. The first two examples would not allocate a separate track for the directory. The third example would result in allocation of 75 tracks for data, plus 1 track for directory space.

   SPACE=(CYL,(5,,10))
 
   SPACE=(TRK,(75,,10))
 
   SPACE=(23200,(150,,10))