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The Xserve RAID architecture is built around one of the overriding principles of digital asset management: the failure of any system component must never be allowed to cause a system-wide failure and loss of data.
The modular design and use of redundant components make Xserve RAID ideal for operations where data availability is crucial. In the event of a component failure, the remote monitoring system notifies the administrator, and repairs can usually be accomplished in seconds without shutting down the system.
Critical Path Eliminated
The Xserve RAID architecture is designed to reduce vulnerability to a component failure. With this in mind, Apple built Xserve RAID around a midplane with a passive data path, a feature not commonly found in other storage systems of its kind. The midplane is the central connector between the drives, RAID controllers, power supplies and cooling modules. Most RAID systems depend on the midplane to relay data and instruction sets between drives, and a failure in the midplane can impair data availability. In Xserve RAID, all data passes through the independent drive channels, which are simply held in place by the midplane.
The multithreaded Xserve RAID system design provides a dedicated ATA drive channel for each of the 14 drive bays. The elimination of interdrive dependencies also enhances availability. Because each hard drive is isolated on its own bus, a drive failure doesn't degrade the accessibility or performance of the surviving drives. In addition, independent drive channels reduce the complexity and cost of high-availability storage, since loop redundancy circuits and signal repeaters aren't required, as in multidrive Fibre Channel and SCSI implementations.
Copilot on Board
The environment management coprocessor monitors the Xserve RAID enclosure and status of system components. If one of the power supplies fails, the other one can simply take over and power the system alone until the failed module is replaced. The coprocessors themselves are redundant: Both monitor the enclosure and record status information on components, enabling automatic adjustments and remote notifications as needed. If a coprocessor fails, the other one can take over monitoring the system.
Hot Sparing
For each RAID controller, any drives not assigned to an array are automatically used as global hot spares. If a drive fails, the RAID controller can automatically rebuild its data on the spare drive without requiring intervention by the administrator. The rebuild operation occurs in the background while the controller processes normal host reads and writes, so that service continues uninterrupted. This gives the administrator ample time to replace the failed drive. Xserve RAID automatically configures the drive as a new hot spare for the array.
