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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 availability. Of course you get all the standard
hardware and RAID options you’d expect. But Xserve RAID goes beyond mere
redundant storage to provide unprecedented reliability in its class. 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 avoid vulnerability to a single point of 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/100
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 drive amplifiers aren’t required, as in multidrive Fibre Channel and
SCSI implementations.Copilot on board The environmental 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. If the enclosure gets too hot or a cooling module fails, the environment manager dynamically adjusts the speed of the remaining cooling module to maintain optimum system temperature. 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 Xserve RAID can be configured with a global hot spare for each RAID controller. 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. |
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AppleCare Service Parts Kits Keep key modules handy to address the most common hardware failures. When you combine this kit with the AppleCare Premium Service and Support plan, Apple experts can often help you troubleshoot and fix your system right over the phone so you don’t have to wait for a technician. 
Hot-swap as neededThe hardware RAID controllers and Apple-designed drive carrier enable true hot-swapping of Xserve RAID drives. A failed drive can actually be removed and replaced without interruption in service or loss of data. The system will continue to operate while the contents of the failed drive are rebuilt on a replacement drive using redundant or parity information. (This feature is not applicable to a RAID 0 configuration, since it doesn’t offer data protection.) 
In addition, the Xserve RAID enclosure is designed for easy serviceability.
All active components are field replaceable and most are hot swappable, including
the power supplies and blower modules.Power backup 
Both controllers have DB-9 serial ports for connecting uninterruptible
power supply (UPS) systems to protect data availability in case of power
outages or electrical surges. For added protection, optional Cache Backup
Battery Modules provide backup power to protect data in the RAID controller
cache for more than 72 hours.
File system journalingXserve RAID benefits from a robust new file system journaling feature in Mac OS X Server that enhances the availability and fault resilience of servers and server-attached storage devices. Journaling protects the integrity of the Mac OS Extended (HFS+) file system in the event of an unplanned shutdown or power failure. It also helps to maximize the uptime of servers and storage devices by dramatically expediting repairs to the affected volumes when the system restarts. |
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RAID 0: Striping. RAID 0 distributes data evenly in horizontal
stripes across an array of drives. While RAID 0 offers substantial speed
enhancements, it provides no data protection: If one drive fails, all data
is lost and all drives must be reformatted.
RAID 1: Mirroring. RAID 1 creates a pair of mirrored drives
with exactly the same data. It provides a high level of data availability
and, by using both drives simultaneously, offers fast read performance.
RAID 0 + 1: Striping over mirroring. This hybrid RAID level
is created by striping data across multiple pairs of mirrored drives. With
Xserve RAID, both striping and mirroring are set up in hardware, which means
the array can run independently of the server processor.
RAID 3: Striping with parity. RAID 3 stripes data across two
or more drives and stores parity data on a dedicated drive. In the event
of a disk failure, the redundant parity bits can be used to reconstruct data
on any drive. 
RAID 5: Striping with distributed parity. For the best balance
of fault tolerance and read/write performance, RAID 5 distributes both data
and parity information across an array of drives one block at a time, with
each drive operating independently. 
RAID 10, 30, and 50: These hybrid RAID levels are created by
combining RAID level 1, 3, or 5 with RAID 0 (RAID 50 refers to RAID 5 + 0).
You create two hardware RAID sets and then stripe the data across the two
sets using software RAID in Mac OS X. | RAID levels RAID technology is based on three practices: Striping to improve storage performance, and mirroring and parity to provide redundancy for increased data protection. Striping divides a logical drive into data blocks, or stripes, that are distributed across the array of physical drives. Data is then laid down according to the stripe paths, so that each file is spread across multiple drives. The simplest method of achieving data redundancy, mirroring involves writing identical copies of all data to a pair of physical drives. A more sophisticated method of creating redundancy, parity provides data protection for an array of drives without requiring complete duplication of the drive contents. Parity information can be used to reconstruct the contents of a failed drive. Each RAID level offers a unique balance of I/O performance, data protection, and storage efficiency. Xserve RAID supports the most popular RAID levels, so you can select the best configuration for your application and your budget. With Xserve RAID, you can combine all 14 drives for high performance at maximum capacity. |
