9
Chapter 1 Introduction
RAID LEVELS (CONT’D)
RAID LEVELS (CONT’D)
RAID Level 5 :
“Independent Access Array with Rotating Parity “High Data Reliability & Transfer Capacity
When RAID Level 5 technology is combined with cache memory to improve its write performance,
the result can be used in any applications where general purpose disks would be suitable.
For read only or read mostly application I/O loads, RAID Level 5 performance should approximate
that of a RAID Level 0 array. In fact, for a given user capacity, RAID Level 5 read performance
should normally be slightly better because requests are spread across one more members than
they would be in a RAID Level 0 array of equivalent usable capacity.
A RAID level 5 array performs best in applications where data and I/O load characteristics match
their capabilities :
•
Data whose enhanced availability is worth protecting, but for which the value of full disk
mirroring is questionable.
•
High read request rates.
•
Small percentage of writes in I/O load.
RAID level 5 arrays have unique performance characteristics:
•
The data can be recalculated or regenerated, using parity, when any drive in the array
fails.
•
When the failed drive is replaced, either automatically if the subsystem contained a hot
spare drive, or by user intervention during a scheduled maintenance period, the system
will be restored its full data redundancy confi guration by rebuilding all of the data that
had been stored on the failed drive onto the new drive. This is accomplished using parity
information and data from the other data disks. Once the rebuild process is complete, all
data is again protected from loss due to any failure of a single disk drive.
P : Parity
Disk 1
Physical
Data
1
2
3
4
5
6
7
8
9
10
11
12
Disk Array Controller
Array Management Software
Provides Logical to Physical Mapping
P
6
9
3
P
8
2
5
P
Disk 2
Disk 3
Disk 4
1
4
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