Apple Servers

Slower Traffic, Keep Right

Xserve RAID features an advanced architecture that sustains storage performance at maximum levels across the entire array. The location or quantity of data on the array doesn't appreciably affect throughput, ensuring consistent, fast data delivery. And the more drives you add, the better the performance.

Slight Drive Train Adjustment

Instead of relying on expensive SCSI or Fibre Channel hard drives, Apple developed a high-throughput Ultra ATA-to-Fibre Channel storage architecture that delivers superior performance and reliability at a much lower cost. Xserve RAID achieves its massive 7TB capacity with 14 hot-swappable 500GB Apple Drive Modules. The high-capacity 7200 RPM Ultra ATA hard drive modules provide affordable expansion and configuration flexibility as storage needs grow.

With 14 independent Ultra ATA drive channels and dual RAID controllers, the Xserve RAID architecture eliminates traditional bottlenecks, maximizing throughput and ensuring fast access to stored data. Support for simultaneous storage processing also allows the performance to scale as capacity increases. Because each hard drive is isolated on its own bus, a drive failure doesn't degrade the accessibility or performance of the surviving drives.

Finely tuned, Independent Motors

The advanced Xserve RAID architecture delivers fast access to storage without compromising data integrity. Dual independent RAID controllers provide protected storage with unprecedented performance. In fact, Xserve RAID boasts a throughput of over 380MB/s, fast enough to support real-time, uncompressed, high-definition (HD) and multiple-stream (SD) video editing without dropping a frame:

Xserve RAID configuration
Usable capacity - RAID 0	1.00TB	3.5TB	7TB
Usable capacity - RAID 5	750GB	3TB	6TB
Video storage in hours (2)
Native DVCPRO HD (1080i60)	14	57	114
Native DVCPRO HD (720p24)	36	140	280
Uncompressed SD 8-bit (3)		39	78
Uncompressed HD 8-bit (1080i60) (3)		6	13

• (1) Testing conducted by Apple in August 2005 using preproduction Xserve RAID systems. Iometer (version 2004.07.30) testing of raw disk throughput on Xserve RAID in both Mac OS X Server v10.4.2 and Windows XP environments has shown the Xserve RAID is capable of delivering up to 192 MB/s on the standard shipping 4 x 250GB disk configuration utilizing a single controller, and an average of over 380MB/s on standard shipping 7 x 500GB and 14 x 500GB raw disk configurations utilizing both RAID controllers. Mac OS X Server v10.4.2 Xserve RAID testing conducted using directly attached Xserve dual processor 2.3 GHz systems; Windows XP Xserve RAID testing conducted using directly attached Dell Precision 670 dual processor 3.6 GHz Xeon systems. Since MBR disks only support partition sizes up to 2TB, sliced arrays were used for Windows XP 14-disk tests.
• (2) Based on formatted RAID 5 capacity in standard shipping configurations. Numbers have been rounded.
• (3) Based on RAID 50.

Apple Drive Modules

Each Apple Drive Module is equipped with 8MB of cache memory for boosting throughput in demanding applications such as video editing. On-drive cache accelerates read and write speeds, but offers less protection in the event of a drive failure. For this reason, drive cache is turned off by default in protected RAID configurations, while video-optimized RAID sets and RAID 0 configurations activate drive cache by default.

The drive controller can access data stored in the drive cache much more quickly than it can read from or write to the actual hard drive platters. Activating drive cache enhances overall system read speeds by allowing the drive controllers to retrieve a block of data before it is required by the host system. The RAID controller provides instructions to each drive in an array, indicating what data block is required next and allowing the individual drive controllers to queue up the data for instant availability.

Redlining It

Similarly, the drive cache serves as a holding tank for data that is being written to the hard drive. When the RAID controller distributes write data to each drive, it can be stored in cache memory, allowing the drive controller to notify the RAID processor immediately that transmission was successful, also known as a "committed write." This frees up the storage pipeline for other needs while the data is being written to the drive platters at maximum speed. Using drive cache is inherently risky: If a drive were to fail during a write operation, the cached data could be lost. The performance needs of video editing and other high-throughput applications outweigh this risk in most situations, but activation of drive cache is not recommended where data protection is a critical requirement.