While RAID 5 is ideally suited for applications with many, small I/O operations, there is no reason why it cannot function equally well for applications with large, sequential I/O operations. This makes RAID 5 an excellent
CAUTION: RAID 5 can withstand a single failure and handle I/O activity without interruption in degraded mode until the failed drive is rebuilt. If a second drive fails while the RAID set is in degraded mode, the entire RAID set will fail.
Just a Bunch of Drives (JBOD)
JBOD makes it possible to connect one or standalone disk drives to the controller. A JBOD disk drive is not part of a redundancy group, even though the controller assigns a redundancy group number to the drive. This number becomes that logical unit number (LUN) that the host will use to address the drive.
One use for JBOD is to connect a system disk drive to the controller. The drive does not become part of a RAID set, but it is made available to the host on the same SCSI bus as the other devices controlled by the controller.
Redundant Operation
When operating in a redundant configuration, the two controllers are linked such that, in case of a failure, the surviving controller can access the other controller’s cache memory and complete all operations that were in progress when the failure occurred. The controllers support two different configurations:
■Active/Active: One host port is active on each controller. The other port on each controller is passive and only used if the peer controller fails.
■Active/Passive: Both host ports on one controller are active. The other controller’s ports are both passive and only used if the primary controller fails.
When one controller fails, the survivor will process all I/O requests until the failed controller is repaired and powered on. The subsystem will then return to its previous state (that is Active/Active or Active/Passive).
