40 Storage Management with DB2 for OS/390
was available.SMS allows the separation of performance and service levelof
data sets by use of the Storage Class.
A Storage Class construct details the intended performance characteristics
requiredfor a data set assigned to a given class. The response times set for each
Storage Class are target responset imes forthe disk controller to achieve when
processing an I/O request. It decides if the volume should be chosen by theus er
or by SMS. Also,it determines whether SMS, when allocating space on the first
volume ofa multi-volume data set, should allocate space on the remaining
volumes as well. Theassignment of a Storage Class does not guarantee its
performance objective,but SMS selects a volume that offers performance as
close as possible.Only SMS managed data sets use Storage Classes. Changes
in a Storage Class appliest othe data sets that are already using thatclass.
StorageClasses can be assigned implicitly through the ACS routines, or explicitly
by using the following:
• JCL statement. The user only needs to specify the relevant Data Class
keyword,such as STORCLAS=SCDBGS.
• DSS COPY and RESTORE commands.
• TSO/E ALLOCATEcommand such as STORCLAS(SCDBGS).
• IDCAMS ALLOCATE and DEFINE commands.
• ISPF/PDF data set allocation panel.
Unlike theData Class, users cannot override individual attribute values when
allocating data sets.
5.3.2.2 Planning for Implementation
For each group of data sets that havesimilar performance objectives, a Storage
Class can exist.To identify and reference a particular Storage Class, a unique
one to eight character name is used, for example,SCDBFAST.
Consideration needsto be given as to whether a user is authorized to select a
specific volumewithin a Storage Group,which is governed by the Guaranteed
Space parameter.This is an arrangement which needs to be agreed upon
between the storage administratorand the D BA.
An exampleof the use of this parameter is in the allocation of the Active logs and
BSDS, where these data sets have critical performan ce and availability
requirements. The DBA should be allowed to allocatet hem on specific volumes,
which is especially important for dual logging capability,to ensure that the logs
areallocated on separate volumes. After being defined, these data sets are rarely
redefined.
Table9 on page 40 provides a list of attributes for consideration.
Table9. Storage Class Attributes
ATTRIBUTE COMMENT
Performanceobjectives -Direct bias
-Direct millisecond response
-Initial access response
-Sequential milliseco ndre sponse
- Sustained data rat e