Performance and Scalability Considerations for Disk Groups

Performance and Scalability Considerations for Disk Groups

• Determining the Number of Disk Groups
• Performance Characteristics When Grouping Disks
• Oracle ASM Storage Limits

Determining the Number of Disk Groups

Use the following criteria to determine the number of disk groups to create:

• Disks in a given disk group should have similar size and performance characteristics. If you have several different types of disks in terms of size and performance, then create several disk groups that contain similar characteristics.
• Create separate disk groups for your database files and fast recovery area for backup files. This configuration allows fast recovery should a disk group failure occur.

Performance Characteristics When Grouping Disks

Oracle ASM load balances the file activity by uniformly distributing file extents across all of the disks in a disk group. For this technique to be effective it is important that disks in a disk group be of similar performance characteristics. For example, the newest and fastest disks might reside in a disk group reserved for the database work area, and slower drives could reside in a disk group reserved for the fast recovery area.

There might be situations where it is acceptable to temporarily have disks of different sizes and performance characteristics coexist in a disk group. This would be the case when migrating from an old set of disks to a new set of disks. The new disks would be added and the old disks dropped. As the old disks are dropped, their storage is migrated to the new disks while the disk group is online.

Oracle ASM Storage Limits

Oracle ASM provides near unlimited capacity for future growth, but does have some storage limits. For example, Oracle ASM has the following limits on the number of disk groups, disks, and files:

• 63 disk groups in a storage system
• 10,000 Oracle ASM disks in a storage system
• 1 million files for each disk group

Without any Oracle Exadata Storage, Oracle ASM has these storage limits:

• 2 terabytes (TB) maximum storage for each Oracle ASM disk
• 20 petabytes (PB) maximum for the storage system

With all Oracle Exadata Storage, Oracle ASM has these storage limits:

• 4 PB maximum storage for each Oracle ASM disk
• 40 exabytes (EB) maximum for the storage system

The maximum size limit of a disk group equals the maximum disk size multiplied by the maximum number of disks in a disk group (10,000).

The maximum number of disks across all disk groups is 10,000. The 10,000 disks can be in one disk group or distributed across a maximum of 63 disk groups. This is a limitation on the number of Oracle ASM disks, not necessarily the number of spindles. A storage array could group multiple spindles into a LUN that is used as a single Oracle ASM disk. However Oracle ASM is currently limited to 2 TB in a single disk unless using Oracle Exadata storage.

File size limits are dependent on the value of the disk group compatibility attributes. Oracle ASM supports file sizes greater than 128 TB in any redundancy mode when the COMPATIBLE.RDBMS disk group attribute is set greater than10.1.

If COMPATIBLE.RDBMS is set to 10.1, the file size limits are less. For example, with COMPATIBLE.RDBMS equal to 10.1 and the AU size equal to 1 MB, Oracle ASM file size limits are:

• External redundancy: 16 TB
• Normal redundancy: 5.8 TB
• High redundancy: 3.9 TB

Note:

Oracle Database supports data file sizes up to 128 TB depending on the file system. In addition, Oracle Database has a file size limit that is dependent on the DB_BLOCK_SIZE initialization parameter.