Availability refers to the storage system's ability to provide user access to data in the case of a hardware or software fault. Midrange systems are classified as highly available because they provide access to data without any single point of failure. However, the following configuration settings can improve performance under degraded mode scenarios.
Single Disk Array Enclosure (DAE) provisioning, which is a disk storage system that contains multiple disk drives, is the practice of restricting the placement of a RAID group within a single enclosure. This is sometimes called horizontal provisioning. Single DAE provisioning is the default method of provisioning RAID groups, and because of its convenience and high availability attributes, it is the most commonly used method. However, you may need to check the vendor configuration for this.
In multiple DAE provisioning, two or more enclosures are used. An example of multiple DAE provisioning requirements is where drives are selected from one or more additional DAEs because there are not enough drives remaining in one enclosure to fully configure the desired RAID group. Another example is SAS backend port balancing. The resulting configuration may or may not span backend ports depending on the storage system model and the drive to enclosure placement.
The following is a typical configuration for multiple DAEs connected to Storage Processors that reside in Disk Processor Enclosure (DPE):
The preceding configuration is considered highly available because if path A fails due to a port, card, or cable failure, the storage processor is still able to talk to the disks in DAE 0 using path B.
If vertical provisioning was used for compelling performance reasons, provision drives within RAID groups to take advantage of request forwarding. This is done as follows:
- RAID 5: At least two (2) drives per SAS backend port in the same DAE
- RAID 6: At least three (3) drives per backend port in the same DAE
- RAID 1/0: Both drives of a mirrored pair on separate backend ports