Control of Where Data Is Stored

A key responsibility of the database server administrator is to control where the database server stores data. By storing high-use access tables or critical dbspaces (root dbspace, physical log, and logical log) on your fastest disk drive, you can improve performance. By storing critical data on separate physical devices, you ensure that when one of the disks holding noncritical data fails, the failure affects only the availability of data on that disk.

As Figure 41 shows, to control the placement of databases or tables, you can use the IN dbspace option of the CREATE DATABASE or CREATE TABLE statements. (For more information, see Tables.)

Figure 41. Controlling Table Placement with the CREATE TABLE... IN Statement

Before you create a database or table in a dbspace, you must first create the dbspace. For more information on how to create a dbspace, see Creating a Dbspace that Uses the Default Page Size.

A dbspace includes one or more chunks, as Figure 42 shows. You can add more chunks at any time. It is a high-priority task of a database server administrator to monitor dbspace chunks for fullness and to anticipate the need to allocate more chunks to a dbspace. (See Monitoring Disk Usage.) When a dbspace contains more than one chunk, you cannot specify the chunk in which the data resides.

Figure 42. Dbspaces That Link Logical and Physical Units of Storage

The database server uses the dbspace to store databases and tables. (See Tables.)

When you create a standard or temporary dbspace, you can specify the page size for the dbspace. You cannot specify a page size for blobspaces, sbspaces, or external spaces. If you do not specify a page size, the size of the root dbspace is the default page size. For more information, see Creating a Dbspace with a Non-Default Page Size.

When you create a standard dbspace, you can specify the first and next extent sizes for the tblspace tblspace in the dbspace. Do this if you want to reduce the number of tblspace tblspace extents and reduce the frequency of situations when you need to place the tblspace tblspace extents in non-primary chunks. For more information, see Specifying the First and Next Extent Sizes for the tblspace tblspace.

You can mirror every chunk in a mirrored dbspace. As soon as the database server allocates a mirror chunk, it flags all space in that mirror chunk as full. See Monitoring Disk Usage.

For information on using ISA or onspaces to perform the following tasks, see Managing Disk Space.

• Creating a dbspace
• Adding a chunk to a dbspace
• Renaming a dbspace
• Dropping a chunk
• Dropping a dbspace, blobspace, or sbspace