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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 10: The Small Computer System Interface, SCSI
      9  RAID

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Summary of Common RAID Levels
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Hardware or Software?

A software-based RAID array carries a lower cost. Software-based RAID can be implemented by the server's operating system or by special software drivers. All RAID functions are carried out in software running on the server CPU. Raid arrays implemented in software occupy system memory, consume CPU cycles, system bus bandwidth, and generally reduce overall server per-formance. A server will not boot if the primary disk of a RAID-1 array has failed.127

A server cannot boot from a software-based RAID-5 array because the oper-ating system must set up the striping and parity on the array and the operating system must be loaded before the data can be read from the RAID-5 array.

Hardware-based RAID is implemented on a RAID controller. Onboard proces-sors and the controller's firmware handle all management of array functions and processes. Hardware controllers can provide notification of disk failures. RAID-5 arrays are bootable with hardware controllers because no additional software is required to read data on the array. Hardware RAID systems do not tax the host CPU as do software systems, so the performance is greatly improved.


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127. http://techdocs.postgresql.org/guides/DiskTuningGuide

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