2.3.3 Resizing Grid Disks

You can resize grid disks and Oracle ASM disk groups to shrink one with excess free space and increase the size of another that is near capacity.

Initial configuration of Oracle Exadata disk group sizes is based on Oracle best practices and the location of the backup files.

• For internal backups: allocation of available space is 40% for the DATA disk groups, and 60% for the RECO disk groups.

• For external backups: allocation of available space is 80% for the DATA disk group, and 20% for the RECO disk group.

The disk group allocations can be changed after deployment. For example, the DATA disk group allocation may be too small at 60%, and need to be resized to 80%.

If your system has no free space available on the cell disks and one disk group, for example RECO, has plenty of free space, then you can resize the RECO disk group to a smaller size and reallocate the free space to the DATA disk group. The free space available after shrinking the RECO disk group is at a non-contiguous offset from the existing space allocations for the DATA disk group. Grid disks can use space anywhere on the cell disks and do not have to be contiguous.

If you are expanding the grid disks and the cell disks already have sufficient space to expand the existing grid disks, then you do not need to first resize an existing disk group. You would skip steps 2 and 3 below where the example shows the RECO disk group and grid disks are shrunk (you should still verify the cell disks have enough free space before growing the DATA grid disks). The amount of free space the administrator should reserve depends on the level of failure coverage.

If you are shrinking the size of the grid disks, you should understand how space is reserved for mirroring. Data is protected by Oracle ASM using normal or high redundancy to create one or two copies of data, which are stored as file extents. These copies are stored in separate failure groups. A failure in one failure group does not affect the mirror copies, so data is still accessible.

When a failure occurs, Oracle ASM re-mirrors, or rebalances, any extents that are not accessible so that redundancy is reestablished. For the re-mirroring process to succeed, sufficient free space must exist in the disk group to allow creation of the new file extent mirror copies. If there is not enough free space, then some extents will not be re-mirrored and the subsequent failure of the other data copies will require the disk group to be restored from backup. Oracle ASM sends an error when a re-mirror process fails due to lack of space.

You must be using Oracle Exadata System Software release 12.1.2.1.0 or higher, or have the patch for bug 19695225 applied to your software.

This procedure for resizing grid disks applies to bare metal and virtual machine (VM) deployments.

• Determine the Amount of Available Space
  To increase the size of the disks in a disk group you must either have unallocated disk space available, or you have to reallocate space currently used by a different disk group.
• Shrink the Oracle ASM Disks in the Donor Disk Group
  If there is no free space available on the cell disks, you can reduce the space used by one disk group to provide additional disk space for a different disk group.
• Shrink the Grid Disks in the Donor Disk Group
  After shrinking the disks in the Oracle ASM disk group, you then shrink the size of the grid disks on each cell.
• Increase the Size of the Grid Disks Using Available Space
  You can increase the size used by the grid disks if there is unallocated disk space either already available, or made available by shrinking the space used by a different Oracle ASM disk group.
• Increase the Size of the Oracle ASM Disks
  You can increase the size used by the Oracle ASM disks after increasing the space allocated to the associated grid disks.