The database needs storage and memory, an obvious statement. This also seems to be what the database administrator is always asking for, more disk and more memory. It is possible with Automatic Storage Management (ASM) to take advantage of any new storage technology and migrate the database by managing the ASM Disk Groups.
ASM Disk Groups should have disks that are the same size and it is not unheard of that size standards might change and makes it difficult to add new disk to the existing disk group. Why does the disk size need to be the same size? Simple answer: For rebalancing. If disks are different sizes, the rebalancing processes are not able to use the full disks or sometimes the rebalance is not as effective.
A few reasons to perform storage migration:
New storage device
Need to move to faster disk (possible SSD)
Maintain disk groups with same disk sizing
So, let’s say we have a disk group DATA1 and it is on a storage array that will be decommission and they have supplied a new storage array. The zoning and mapping of the disk devices will have to be allocated, visible and permissioned for the ASM instance. If the current disk group is using disk that are 50GB in size, it is possible that the new devices are 100GB as an example, and the disk can be added with the plan that all of the disks will not be 100GB.
The new disk is added to the disk group, and disk is rebalanced so that there is data on all of the disks.
Adding disks would be using just the ALTER DISKGROUP and a REBALANCE POWER can be set:
ALTER DISKGROUP data1 ADD DISK ‘/devices_new/disk2*’ REBALANCE POWER 4;
When adding disk, keep in mind that rebalancing does add an additional workload on the disk, which will mean that it is important to perform these tasks in a maintenance window. The database will still be available, so it is not an outage but it should be during a non-peak period.
After the disk is added and rebalanced, then the disk can be dropped. Check the V$ASM_OPERATION view for information about the timing of the rebalancing.
Removing disks will leave the new storage as needed then the decommissioning process can be used to remove the storage from the ASM server.
ALTER DISKGROUP data1 DROP DISK ‘/devices_old/disk1a’ REBALANCE POWER 8;
View V$ASM_DISK to verify the disks that are part of the disk group.
Using ASM ADD and DROP disk from the disk group will allow for online migration to different storage. The caution is to perform the ADD, DROP and REBALANCE in non-peak times for better performance of the migration and continue to allow the database performance for users to remain constant. Using the REBALANCE POWER can also be adjusted to better performance for the migration.