Apple Mac OS X Server (Version 10.6 Snow Leopard) - Manual (Page 160)

Some single points of failure include:
Computer system Â
Hard disk Â
Power supply Â
Although it is almost impossible to eliminate all single points of failure, you should
minimize them as much as possible. For example, using a backup computer and a file
storage pool for Mac OS X Server eliminates the computer as a single point of failure.
Although master and backup computers can fail at once or one after the other, the
possibility of such an event happening is negligible.
Another way to prevent a computer from failing is to use a backup power source and
take advantage of hardware RAID to mirror the hard disk. With hardware RAID, if the
main disk fails, the system can still access the same data on the mirror drive, as is the
case with Xserve.
Using Xserve for High Availability
Xserve is designed for extra reliability and hence, high availability.
Although you can use desktop systems like the Mac Pro to provide Mac OS X Server
services very reliably, Xserve has the following additional features that make it ideal for
high availability situations.
Xserve has eight fans. In the case of a single fan failure, the other fans speed up to Â
compensate, allowing your server to keep running.
An independent drive architecture isolates the drives electrically, preventing a
Â
single drive failure from causing unavailability or performance degradation of the
surviving drives—a common problem with multidrive SCSI implementations.
Xserve uses Error Correction Code (ECC) logic to protect the system from corrupt
Â
data and transmission errors.
Each DIMM has an extra memory module that stores checksum data for every
transaction. The system controller uses this ECC data to identify single-bit errors and
corrects them on the fly, preventing unplanned system shutdowns.
In the rare event of multiple-bit errors, the system controller detects the error
and triggers a system notification to prevent bad data from corrupting further
operations.
You can set the Server Monitor software to alert you if error rates exceed the
defined threshold.
Xserve has built-in hardware RAID mirroring, which protects your server from failing
Â
if the main drive fails.
For more information about Xserve, visit www.apple.com/xserve/.
160 Chapter 7 Ongoing System Management