Data migration that results from the QoS Plus feature occurs when the system, after an analysis of the historical usage information for the block of data, determines that another Storage Class or RAID level can better service that block of data.
When optimizing for system performance,
QoS Plus moves frequently accessed data blocks of a LUN to a faster, more expensive Storage Class, such as high-performance solid-state drives (SSDs). Conversely, when optimizing for cost,
QoS Plus moves infrequently accessed data blocks to a slower, less expensive Storage Class, such as high-capacity hard disk drives (HDDs).
Note: When data migration occurs, the Oracle FS System gives the higher priority volumes a greater opportunity to occupy the higher performance Storage Classes. To migrate the data, the system initiates a task that runs as a background process.
You can limit data migration to use only designated Storage Classes. When you create a LUN using QoS Plus, you can specify the Storage Classes to include for data migration and to exclude for data migration. QoS Plus moves infrequently accessed data to a lower performance Storage Class, such as Capacity Disk.
Excluding less expensive Storage Classes and including high performance Storage Classes causes immobile storage capacity. Immobile data occurs when the system cannot migrate the data to a lower Storage Class. Data that cannot migrate to a lower Storage Class is considered immobile and the storage capacity that this data occupies is immobile. Data migration is efficient when 100% of the system capacity is available.
Note: If the preferred Storage Class is full, data might be placed temporarily in a non-preferred Storage Class.
Provide sufficient storage for data migration and allow the system to use all of that storage capacity. Oracle FS System Manager (GUI) provides a graph that shows all of your allocated storage capacities and immobile storage capacities from the Storage Domain page.