HP 226593-B21 Smart Array 5i Plus Controller and Battery Backed Write Cache En - Page 89

Disadvantages, RAID 5—Distributed Data Guarding

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Drive Arrays and Fault Tolerance Disadvantages The disadvantages of RAID 1+0 are that: • It is expensive (many drives are needed for fault tolerance). • Only 50% of total drive capacity useable for data storage. RAID 5-Distributed Data Guarding By this method, a block of parity data is calculated for each stripe from the data that is in all other blocks within that stripe. The blocks of parity data are distributed over every physical drive within the logical drive (see Figure C-7). When a physical drive fails, data that was on the failed drive can be calculated from the user data on the remaining drives and the parity data. This recovered data is usually written to an online spare in a process called a rebuild. This configuration is useful when cost, performance, and data availability are equally important. S1 B1 S2 B3 S3 P5,6 S4 B7 B2 P3,4 B5 B8 P1,2 B4 B6 P7,8 Figure C-7: Distributed data guarding, showing parity information (Px,y) C-8 Compaq Smart Array 5i Plus Controller and Battery Backed Write Cache Enabler User Guide

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Drive Arrays and Fault Tolerance
C-8
Compaq Smart Array 5i Plus Controller and Battery Backed Write Cache Enabler User Guide
Disadvantages
The disadvantages of RAID 1+0 are that:
•
It is expensive (many drives are needed for fault tolerance).
•
Only 50% of total drive capacity useable for data storage.
RAID 5—Distributed Data Guarding
By this method, a block of
parity data
is calculated for each stripe from the data that
is in all other blocks within that stripe. The blocks of parity data are distributed over
every physical drive within the logical drive (see Figure C-7). When a physical drive
fails, data that was on the failed drive can be calculated from the user data on the
remaining drives and the parity data. This recovered data is usually written to an
online spare in a process called a
rebuild.
This configuration is useful when cost, performance, and data availability are equally
important.
S1
S2
S3
S4
B1
B3
P5,6
P3,4
P1,2
P7,8
B7
B2
B5
B8
B4
B6
Figure C-7:
Distributed data guarding, showing parity
information (Px,y)