Your plans for the provisioning of logical volumes must take into account the extra capacity that the Oracle FS System allocates to accommodate various needs. Together, these extra capacities comprise what is called system overhead.
Oracle FS System Manager (GUI) reports the physical capacities for the Storage Domains, for the drive groups, and for the logical volumes that exist in an Oracle FS System.
Overhead for RAID Protection
To account for the data protection overhead, the
Oracle FS System increases the requested capacity by a certain amount when creating a volume. The amount of additional capacity depends on the RAID level of the volume that is being created and on the initial Storage Class that is selected for the volume:
RAID 5, solid state drive (SSD): 20%
RAID 5, hard disk drive (HDD): 10%
Note: In the GUI, the physical capacity and the logical capacity for RAID 5 HDDs appears to be 20% because the system includes the spare space in the overhead calculation.
RAID 6, capacity HDD: 20%
RAID 10, all Storage Classes: 100%
For example, assume that you request 250 GB of capacity for a new volume. If you select single redundancy on any HDD Storage Class, the system uses a RAID 5 geometry for the new volume and allocates an additional 10% (25 GB) of capacity. If, however, you select double redundancy on capacity HDDs, the system uses a RAID 6 geometry for the new volume and allocates an additional 20% (50 GB) of capacity.
Note: These overhead percentages are approximate and can vary somewhat for different reasons.
The capacity that a volume consumes and the capacity that the system reports for the volume both include the overhead for RAID protection.
Overhead for Infill Reserve
For each Storage Domain that contains hard disk drives (HDDs), the
Oracle FS System reserves 50 GB of physical capacity as a reserve that can be used, if needed, for infilling thin volumes. This reserve is available to all SAN LUNs that are located partly or wholly on those HDDs.
Note: The system includes the infill reserve capacity in the free physical capacities that are reported for the Storage Domains and for the drive groups.
The Oracle FS System reserves this infill reserve capacity to help prevent an inadvertent exhaustion of the physical capacity when thinly provisioned volumes grow. When a thinly provisioned volume needs to grow, the system uses this reserve capacity when all of the regular HDD capacity in the Storage Domain has been allocated.