9.2.1.2 Sparse Disk Groups

Exadata Snapshots utilize Oracle ASM sparse disk groups.

Sparse data files can be created only in Oracle Automatic Storage Management (Oracle ASM) sparse disk groups. The following figure shows a sparse disk group containing three Exadata Snapshots.

Figure 9-5 Sparse Disk Group Contains Exadata Snapshots

Description of "Figure 9-5 Sparse Disk Group Contains Exadata Snapshots"

A sparse disk group has the following attributes:

• compatible.asm must be set to 12.1.0.2 or higher.
• compatible.rdbms must be set to 12.1.0.2 or higher.
• cell.smart_scan_capable must be set to true.
• cell.sparse_dg must be set to allsparse. This attribute identifies the disk group to Oracle ASM as being made up of sparse grid disks.
• As is the case for all Oracle ASM disk groups on Exadata, the recommended allocation unit (AU) size is 4M.

For example, the following SQL command creates a sparse disk group:

SQL> CREATE DISKGROUP SPARSE
  NORMAL REDUNDANCY 
  DISK 'o/*/SPARSE_*'
  ATTRIBUTE 
    'compatible.asm'          = '12.1.0.2',
    'compatible.rdbms'        = '12.1.0.2',
    'cell.smart_scan_capable' = 'true',
    'cell.sparse_dg'          = 'allsparse',
    'au_size'                 = '4M';

A sparse Oracle ASM disk group can store both sparse and non-sparse files, which makes it possible to store a test master database and Exadata Snapshots in the same disk group. However, because sparse grid disks are limited in size (to a maximum of 4 TB on a cell disk) and because there is no benefit in placing non-sparse files in a sparse disk group, you should reserve space in sparse disk groups for the sparse files associated with Exadata Snapshots and place the test master database in a regular (non-sparse) disk group.