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StorCase Technology Fibre-to-SCSI Single RAID User Manual

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S10C100 User's Guide - Rev. A01

StorCase Technology, Inc.

Appendix B - Array Basics

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RAID 5

RAID 5 arrays contain redundant information in the form of parity data, which is calculated

block-by-block for all user data. The parity information is distributed across the disks in the

array (similar to RAID 0 arrays) and occupies the equivalent capacity of about one disk. Data

is interspersed with the parity information. If one disk in the array fails, the data on the failed

disk can be reconstructed from the parity data and user data on the remaining disks. Two disks

must fail before the entire array fails.

The read performance of a RAID 5 array is excellent, comparable to that of a RAID 0 array.

Write performance is lower than that of a RAID 0 array, because write operation involves

calculating and writing new parity data as well as writing the new user data.

RAID 50

RAID 50 arrays contain redundant information in the form of parity data, which is calculated

block-by-block for all user data. The partity information is distributed across the disks in the

array and occupies the equivalent capacity of approximately one disk. Data is interspersed

with the parity information. If one disk in the array fails, the data on the failed disk can be

reconstructed from the parity data and user data on the remaining disks. Two disks must fail

before the entire array fails.

The read performance of a RAID 5 array is excellent - comparable to that of a RAID 0 array.

Write performance is lower than that of a RAID 0 array, because write operations involve

calculating and writing new parity data, as well as writing the new user data.

Volume Sets

A volume set is a host-accessible LUN that maps to a single disk in the array, similar to JBOD.

Volume sets are non-redundant and have a capacity slightly less than the physical disk they

are created from. Volume sets are useful if a single disk is available and not used as a spare.

Mixing Disks from Different Manufacturers or with Different Capacities

An array can contain different models of SCSI disks, even disks with different capacities (an

array can include a 4GB disk and a 9GB disk). However, if disks with different capacities are

mixed, the logical capacity of all other disks in the array is determined by the smallest disk

(regardless of RAID level). To maximize disk capacity, use disks of similar size.