Specifications
16
Technology Overview
Xserve RAID
Monitoring of drive health
Xserve RAID hardware and remote management software work together to provide
industry-leading monitoring and alerting capabilities. The RAID controller automati-
cally reads Self-Monitoring, Analysis, and Reporting Technology (SMART) data, which
allows each hard drive to report its health. This enables the operating system to
warn the administrator of a prefailure condition, providing the opportunity to replace
the hard drive before the failure occurs. Each drive module has two LEDs for local
monitoring of drive activity and health.
Passive Midplane Data Path
The Xserve RAID architecture is designed to avoid vulnerability to a single point of
failure. This means that the failure of any single component can’t result in a system-
wide failure and loss of data availability. For highest reliability, Apple built Xserve
RAID around a midplane that serves as a passive data path. 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. This design improves system reliability and
protects the availability of stored data.
RAID Controllers
Xserve RAID features two controllers that independently manage storage operations
for a set of up to seven drives. Dual independent controllers allow for simultane-
ous storage processing. Because each controller needs to manage only seven drives,
Xserve RAID delivers outstanding performance that scales as capacity increases. These
controller modules oer advanced availability and manageability features, including
the environment manager, which manages RAID functions and monitors status and
activity of system components.
RAID processor
Each Xserve RAID controller has a powerful processor that manages all the data
transfers between the hard drives and host computer. It performs the complex parity
calculations required for RAID levels 3 and 5, generating parity data on the y while
managing write operations to each drive in the set. In the event of a drive failure,
the RAID processor uses redundant data to rebuild its contents on a spare drive.
5
Xserve RAID supports RAID levels 0, 1, 3, 5, and 0+1 using the RAID processor, as well
as hybrid RAID levels 10, 30, and 50 by combining hardware RAID capabilities with
host-based software RAID.
An innovative process ensures that RAID sets are error-free and tuned for maximum
throughput. When creating a RAID set, Xserve RAID veries the integrity of the array
by checking every block on every disk for potential errors. If a bad block is discovered,
it simply chooses another block that’s in close proximity to the original—eliminating
unnecessary head movement that can result in lower performance.
To maintain optimal throughput, the RAID processor uses another special technique
that retrieves data on the rst revolution of the RAID set. This avoids multiple retries
(an activity that can hamper performance on RAID systems), while allowing Xserve
RAID to maintain a nearly constant sustained throughput over the entire platter
of each hard disk. Outer and inner tracks perform comparably, providing consistent,
fast delivery of data to the host system. This is particularly critical in high-bandwidth
applications such as HD video editing, which requires fast storage throughput for
clean video capture and smooth playback.