DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

5.2.1 Conclusion
If MGEXTSZ is set to YES, DB2 will always honor SECQTY value specified by the user in the
Catalog if it is greater than the secondary quantity value calculated by the sliding scale
methodology used by the solution. This allows the user to override with a larger SECQTY
value to get to maximum data set size quicker.
If no SECQTY value is specified by the user, it will be recorded as -1 in the associated SQTY
column in SYSTABLEPART or SYSINDEXPART catalog table, DB2 will calculate 10% of the
PRIQTY value specified by the user in the Catalog but will cap by the maximum secondary
quantity allocation according to the respective sliding scale.
If you specify SECQTY 0, it will be honored by DB2. This results in SQLCODE -904 and
reason code 00D70027 when DB2 tries to extend the data set.
For a striped data set, the proposed extent size will have to be divided by the number of
stripes and each stripe can have up to 255 extents.
When extent consolidation is in effect in z/OS V1.5 the secondary space allocation quantity
can be smaller than specified or calculated by the sliding scale based on the physical extent
number. PTF UQ89517 for APAR PQ88665 corrects this problem by using the logical extent
number together with the sliding scale.
No primary nor secondary allocation needs to be specified at create time. DB2 determines
the optimal values based on a combination of the DSSIZE, LARGE and PIECESIZE
specifications and system parameters MGEXTSZ, TSQTY and IXQTY. The secondary
allocation is automatically increased using a sliding scale until 127 extents and a constant
size thereafter to avoid exhausting the maximum number of extents per data sets.
The enhancement has the potential to improve SQL SELECT and INSERT performance,
more efficient preformat, prefetch, and deferred write processing.
This enhancement changes the PRIQTY default to achieve cylinder allocation, forces tiny
SECQTY allocations to use these defaults, and slides secondary quantity allocations
upwards.
Any penalty with having multiple extents goes down as the extent size increases. The size of
the extents is actually more important than the actual number of extents. Above an extent size
of 10 cylinders independent of the number of extents there is no noticeable difference in
performance.
Note that no space availability nor disk fragmentation is addressed by this enhancement, and
still remain the responsibility of DFSMS and the storage administrator.
5.2.2 Recommendations
We recommend to start with activating automatic space management for secondary extents
(MGEXTSZ=YES). Availability will increase by avoiding out of space abends due to maximum
number of extents reached and, as I/Os will never span extents, your installation will benefit
from better extent sizing when performing
► Prefetch
► Deferred write
► Mass Insert
► Utility Processing
Chapter 5. Availability and capacity enhancements 229