Rules for primary and secondary space allocation

You can specify the primary and secondary space allocation or let Db2 choose them. Having Db2 choose the values, especially the secondary space quantity, increases the possibility of reaching the maximum data set size before running out of extents.

In the following rules that describe how allocation works, these terms are used:
PRIQTY, SECQTY
The keywords for CREATE TABLESPACE, ALTER TABLESPACE, CREATE INDEX, and ALTER INDEX.
specified-priqty
The user-specified value for PRIQTY.
specified-secqty
The user-specified value for SECQTY.
actual-priqty
The actual primary space allocation, in kilobytes.
actual-priqty-cylinders
The actual primary space allocation, in cylinders.
actual-secqty
The actual secondary space allocation, in kilobytes.
actual-secqty-cylinders
The actual secondary space allocation, in cylinders.
calculated-extent-cylinders
A value that is calculated by Db2 using a sliding scale. A sliding scale means that the first secondary extent allocations are smaller than later secondary allocations. For example, Figure 1 shows the sliding scale of secondary extent allocations that Db2 uses for a 64-GB data set. The size of each secondary extent is larger for each secondary extent that is allocated up to the 127th extent. For the 127th secondary extent and any subsequent extents, the secondary size allocation is 559 cylinders.
Figure 1. Sliding scale allocation of secondary extents for a 64 GB data set
Begin figure description. A graph shows extent number on the x-axis and extent size in cylinders on the y-axis. An upwardly sloping line levels out at 559 at the 127th extent. End figure description.

Rules

Rule 1: (for primary space allocation)
If PRIQTY is specified and specified-priqty is not equal to -1, actual-priqty is at least specified-priqty KB.
Start of changeIf PRIQTY is not specified or specified-priqty is equal to -1, actual-priqty is determined as follows:
  • For a table space, if the TSQTY subsystem parameter value is specified and is greater than 0, actual-priqty is at least the value of TSQTY.

    If the TSQTY subsystem parameter is not specified or is 0, actual-priqty is one cylinder for a non-LOB table space. actual-priqty is 10 cylinders for a LOB table space.

  • For an index, if the IXQTY subsystem parameter value is specified and is greater than 0, actual-priqty is at least the value of IXQTY.

    If the IXQTY subsystem parameter is not specified or is 0, actual-priqty is one cylinder.

End of change
Rule 2: (for secondary space allocation)
If SECQTY is not specified, the following formulas determine actual-secqty:
  • If the maximum size of a data set in the table space or index is less than 32 GB, the formula is: 
    actual-secqty-cylinders=
MAX(0.1*actual-priqty-cylinders, MIN(calculated-extent-cylinders, 127))
  • If the maximum size of a data set in the table space or index is 32 GB or greater, the formula is: 
    actual-secqty-cylinders=
MAX(0.1*actual-priqty-cylinders, MIN(calculated-extent-cylinders, 559))
Rule 3 (for secondary space allocation)
If SECQTY is 0, actual-secqty is 0.
Rule 4 (for secondary space allocation)
If SECQTY is specified and specified-secqty is not equal to -1 or 0, the following formulas determine actual-secqty. If the maximum size of a data set in the table space or index is less 32 GB, the formula is: 
actual-secqty-cylinders=
MAX(MIN(calculated-extent-cylinders, 127),specified-secqty-cylinders)
If the maximum size of a data set in the table space or index is 32 GB or greater, the formula is: 
actual-secqty-cylinders=
MAX(MIN(calculated-extent-cylinders, 559),specified-secqty-cylinders)
Rule 5 (for secondary space allocation):
When a table space requires a new piece, the primary allocation quantity of the new piece is determined by using the maximum of the following values:
  • The quantity that is calculated through sliding scale methodology
  • The primary quantity from rule 1
  • The specified SECQTY value