DB2 storage groups

DB2® storage groups are a set of volumes on disks that hold the data sets in which tables and indexes are stored.

The description of a storage group names the group and identifies its volumes and the VSAM (Virtual Storage Access Method) catalog that records the data sets. The default storage group, SYSDEFLT, is created when you install DB2.

Within the storage group, DB2 does the following actions:

• Allocates storage for table spaces and indexes
• Defines the necessary VSAM data sets
• Extends and deletes VSAM data sets
• Alters VSAM data sets

All volumes of a given storage group must have the same device type. However, parts of a single database can be stored in different storage groups.

DB2 can manage the auxiliary storage requirements of a database by using DB2 storage groups. Data sets in these DB2 storage groups are called "DB2-managed data sets."

These DB2 storage groups are not the same as storage groups that are defined by the DFSMS storage management subsystem (DFSMSsms).

You have several options for managing DB2 data sets:

• Let DB2 manage the data sets. This option means less work for DB2 database administrators.

  After you define a DB2 storage group, DB2 stores information about it in the DB2 catalog. (This catalog is not the same as the integrated catalog facility catalog that describes DB2 VSAM data sets). The catalog table SYSIBM.SYSSTOGROUP has a row for each storage group, and SYSIBM.SYSVOLUMES has a row for each volume. With the proper authorization, you can retrieve the catalog information about DB2 storage groups by using SQL statements.

  When you create table spaces and indexes, you name the storage group from which space is to be allocated. You can also assign an entire database to a storage group. Try to assign frequently accessed objects (indexes, for example) to fast devices, and assign seldom-used tables to slower devices. This approach to choosing storage groups improves performance.

  If you are authorized and do not take specific steps to manage your own storage, you can still define tables, indexes, table spaces, and databases. A default storage group, SYSDEFLT, is defined when DB2 is installed. DB2 uses SYSDEFLT to allocate the necessary auxiliary storage. Information about SYSDEFLT, as with any other storage group, is kept in the catalog tables SYSIBM.SYSSTOGROUP and SYSIBM.SYSVOLUMES.

  For both user-managed and DB2-managed data sets, you need at least one integrated catalog facility (ICF) catalog; this catalog can be either a user catalog or a master catalog. These catalogs are created with the ICF. You must identify the catalog of the ICF when you create a storage group or when you create a table space that does not use storage groups.

• Let SMS manage some or all the data sets, either when you use DB2 storage groups or when you use data sets that you have defined yourself. This option offers a reduced workload for DB2 database administrators and storage administrators. You can specify SMS classes when you create or alter a storage group.
• Define and manage your own data sets using VSAM Access Method Services. This option gives you the most control over the physical storage of tables and indexes.