External Controller Based
An external controller based RAID array holds several advantages over a bus-based system.129 The highest I/O performance can be obtained with an exter-nal controller based RAID arrangement. An external RAID subsystem is resistant to failure of the host server. Bottlenecks at the host server PCI bus can be minimized.
External controller RAID subsystems are enclosed in a separate enclosure. This can provide several features not available with an internal controller setup. Onboard battery backup of the external host controller can maintain power for controller cache memory until the data can be written to the disk subsystem. Most enclosures provide for redundant power supplies and cool-ing fans. Hot swap capabilities (replacing components without powering down the main system) can enhance the fault tolerance of the disk subsys-tem.
SCSI Enclosure Services (SES) is an ANSI standard that defines how the en-closure status information is communicated across the SCSI bus. SES uses in-band reporting. The SES specification outlines the commands and pa-rameters to manage and report the status of power supplies, cooling fans, displays and indicators, and individual drives. This can be invaluable when managing a large mission critical SCSI installation.
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