Page 182 - DCAP403_Operating System

Unit 9: I/O & Secondary Storage Structure

Concatenated Array: A Concatenated array is NOT RAID, although it is an array. It is a group of Notes
disks connected together, end-to-end, for the purpose of creating a larger logical disk. Although
it is not RAID, it is included here as it is the result of early attempts to combine multiple disks
into a single logical device. There is no redundancy with a Concatenated array. Any performance

improvement over a single disk is achieved because the file-system uses multiple disks. This type
of array is usually slower than a RAID-0 array of the same number of disks.
The good point of a Concatenated array is that different sized disks can be used in their entirety.
The RAID arrays below require that the disks that make up the RAID array be the same size, or
that the size of the smallest disk be used for all the disks.
The individual disks in a concatenated array are organized as follows:
Figure 9.17 Concatenated Segments

Drive 0

Drive 1
Drive 2

Drive 3

RAID-0: In RAID Level 0 (also called striping), each segment is written to a different disk, until
all drives in the array have been written to.

The I/O performance of a RAID-0 array is significantly better than a single disk. This is true on
small I/O requests, as several can be processed simultaneously, and for large requests, as multiple
disk drives can become involved in the operation. Spindle-sync will improve the performance for
large I/O requests.
This level of RAID is the only one with no redundancy. If one disk in the array fails, data is lost.
The individual segments in a 4-wide RAID-0 array are organized as follows:

Table 9.1: RAID-0 Segments
Drive 0 Drive 1 Drive 2 Drive 3
0 1 2 3
4 5 6 7
8 9 10 11
12 13 14 15
16 17 18 19
20 21 22 23

RAID-1: In RAID Level 1 (also called mirroring), each disk is an exact duplicate of all other
disks in the array. When a write is performed, it is sent to all disks in the array. When a read is
performed, it is only sent to one disk. This is the least space efficient of the RAID levels.

A RAID-1 array normally contains two disk drives. This will give adequate protection against
drive failure. It is possible to use more drives in a RAID-1 array, but the overall reliability will
not be signifi cantly effected.
RAID-1 arrays with multiple mirrors are often used to improve performance in situations where
the data on the disks is being read from multiple programs or threads at the same time. By being
able to read from the multiple mirrors at the same time, the data throughput is increased, thus
improving performance. The most common use of RAID-1 with multiple mirrors is to improve
performance of databases.

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