Specifications

17
Technology Overview
Xserve RAID
Controller cache memory
Each RAID controller is equipped with 512MB of cache memory for enhanced storage
throughput. For optimal read performance, streaming data can be queued into cache,
and sophisticated algorithms measure data trac patterns and reallocate read cache
dynamically. Depending on write performance and data protection requirements,
cache memory can be congured as “write-back” or “write-through.” In either case,
the write cache conguration does not aect read cache settings.
Write-back cache. For performance-critical applications, cache memory can accelerate
write speeds using a conguration called write-back cache. In this mode, data is
considered committed, or successfully received, as soon as the RAID controller writes
back to the host that the information is stored in cache memory. However, as with
drive cache, write-back cache is vulnerable to data loss. Since cached data is stored
in SDRAM, which requires a continuous supply of power, a power outage or major
system failure could result in data loss. This risk can be minimized by connecting a
UPS system or using the optional Cache Backup Battery Modules. With a minimum
of 72 hours of power, the battery modules preserve the contents of cache memory
until system power is restored.
Write-through cache. When write-through cache is enabled, the RAID controller
writes data through the cache, directly to the disks, before informing the host that
the write is committed. Although this may reduce performance, write-through cache
is the recommended setting for protected RAID without a battery or UPS device. For
high-bandwidth applications or when multiple servers are attached to Xserve RAID,
write-back cache is recommended. If a power outage occurs while using write-back
cache with a UPS system, Xserve RAID automatically reverts to write-through cache
to protect the data in case of a UPS failure.
Environment managers
Each Xserve RAID controller module contains a special coprocessor that monitors
the enclosure environment and manages conguration of the main processor’s RAID
functions. This innovative environment manager simplies management and boosts
reliability by ensuring continuous feedback on the status and activity levels of system
components.
The environment managers integrate seamlessly with RAID Admin software and the
RAID processor, providing advanced capabilities such as RAID Now for background
initialization during RAID creation. Each environment manager operates on an indepen-
dent bus and communicates with the host system via Ethernet. All the management
and monitoring processes are handled “out of band,” which means Fibre Channel
bandwidth is not specied for management chores.
Each environment manager communicates with both RAID controllers and all hard
drives. If one of the RAID environment managers fails, the other takes over and continues
the monitoring and management functions to avoid any interruption in service. At the
same time, it immediately noties the administrator of the component failure via email.
Because the RAID environment managers connect to the host computer over Ethernet
using standard Internet protocols, it’s easy to connect to and monitor Xserve RAID
systems from any computer on the Internet. In addition to an Ethernet port, each
environment manager has a serial port for connecting an optional UPS device,
providing an extra measure of protection against power surges or electrical outages.