RAID (Redundant Array of Independent Drives (or
Disks)
A. A Redundant Array of Independent Drives (or Disks), also known as
Redundant Array of Inexpensive Drives (or Disks) (RAID) is an term for data
storage schemes that divide and/or replicate data among multiple hard drives.
RAID can be designed to provide increased data reliability or increased I/O
performance, though one goal may compromise the other.
Mainly there are 3 types of RAID :
RAID 0
RAID 1
RAID 5
RAID 0 (Striping)
This level is achieved by grouping 2 or more hard disks into a single unit
with the total size equaling that of all disks used. Practical example: 3
disks, each 80GB in size can be used in a 240GB RAID 0 configuration.
RAID 0 works by breaking data into fragments and writing to all disk
simultaneously. This significantly improves the read and write performance. On
the other hand, no single disk contains the entire information for any bit of
data committed. This means that if one of the disks fails, the entire RAID is
rendered inoperable, with unrecoverable loss of data.
RAID 0 is suitable for non-critical operations that require good
performance, like the system partition or the /tmp partition where lots of
temporary data is constantly written. It is not suitable for data storage.
RAID 1 (Mirroring)
This level is achieved by grouping 2 or more hard disks into a single unit
with the total size equaling that of the smallest of disks used. This is
because RAID 1 keeps every bit of data replicated on each of its devices in the
exactly same fashion, create identical clones. Hence the name, mirroring.
Practical example: 2 disks, each 80GB in size can be used in a 80GB RAID 1
configuration. On a side note, in mathematical terms, RAID 1 is an AND
function, whereas RAID 0 is an OR.
Because of its configuration, RAID 1 reduced write performance, as every
chunk of data has to be written n times, on each of the paired devices. The
read performance is identical to single disks. Redundancy is improved, as the
normal operation of the system can be maintained as long as any one disk is
functional. RAID 1 is suitable for data storage, especially with non-intensive
I/O tasks.
RAID 5
This is a more complex solution, with a minimum of three devices used. Two
or more devices are configured in a RAID 0 setup, while the third (or last)
device is a parity device. If one of the RAID 0 devices malfunctions, the array
will continue operating, using the parity device as a backup. The failure will
be transparent to the user, save for the reduced performance.
RAID 5 improves the write performance, as well as redundancy and is useful
in mission-critical scenarios, where both good throughput and data integrity
are important. RAID 5 does induce a slight CPU penalty due to parity
calculations.
PRACTILE OF RAID USING RAID 1
First create 3
partitions sda10,sda11,sda12 of raid type
[root@www ~]#
mdadm -C /dev/md0 -l 1 -n 2 /dev/md10 /dev/md11
C-- create
l-- level
n-- numder of
device
[root@www ~]#
mdadm --detail /dev/md0
[root@www ~]# mdadm --detail /dev/md0
/dev/md0:
Version : 00.90.03
Creation Time : Sun Jan 13 22:11:49 2013
Raid Level : raid1
Array Size : 104320 (101.89 MiB 106.82 MB)
Used
Dev Size : 104320 (101.89 MiB 106.82 MB)
Raid
Devices : 2
Total
Devices : 2
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Sun Jan 13 22:24:07 2013
State : clean
Active
Devices : 2
Working Devices : 2
Failed
Devices : 0
Spare
Devices : 0
UUID : 63a6d6db:ced50964:3fb69cfe:6b30dd55
Events : 0.6
Number Major Minor
RaidDevice State
0 8 10
0 active sync /dev/sda10
1 8 12
1 active sync /dev/sda11
[root@www ~]#
mkfs.ext3 /dev/md0
[root@www ~]#
mkdir /raid
[root@www ~]#
mount /dev/md0 /raid
now add a spare
device
[root@www ~]#
mdadm --add /dev/md0 /dev/md12
/dev/md0:
Version : 00.90.03
Creation Time : Sun Jan 13 22:11:49 2013
Raid Level : raid1
Array Size : 104320 (101.89 MiB 106.82 MB)
Used
Dev Size : 104320 (101.89 MiB 106.82 MB)
Raid
Devices : 2
Total
Devices : 3
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Sun Jan 13 22:24:07 2013
State : clean
Active
Devices : 2
Working Devices : 3
Failed
Devices : 0
Spare
Devices : 1
UUID : 63a6d6db:ced50964:3fb69cfe:6b30dd55
Events : 0.6
Number Major Minor
RaidDevice State
0 8 10
0 active sync /dev/sda10
1 8 11
1 active sync /dev/sda11
2
8 12 2
spare sync /dev/sda12
[root@www ~]# cp
-rvf /etc /raid
[root@www ~]#
mdadm --fail /dev/md0 /dev/sda11
[root@www ~]#
Used Dev Size : 104320 (101.89 MiB 106.82 MB)
Raid
Devices : 2
Total
Devices : 3
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Sun Jan 13 22:23:54 2013
State : clean, degraded, recovering
Active
Devices : 1
Working Devices : 2
Failed
Devices : 1
Spare
Devices : 1
Rebuild Status : 50% complete
UUID : 63a6d6db:ced50964:3fb69cfe:6b30dd55
Events : 0.4
Number Major Minor
RaidDevice State
0 8 10
0 active sync /dev/sda10
2 8 12
1 spare rebuilding /dev/sda12
3 8 11
- faulty spare /dev/sda11
[root@www ~]# mdadm --detail /dev/md0
/dev/md0:
Version : 00.90.03
Creation Time : Sun Jan 13 22:11:49 2013
Raid Level : raid1
Array Size : 104320 (101.89 MiB 106.82 MB)
Used
Dev Size : 104320 (101.89 MiB 106.82 MB)
Raid
Devices : 2
Total
Devices : 3
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Sun Jan 13 22:23:54 2013
State : clean, degraded, recovering
Active
Devices : 1
Working Devices : 2
Failed
Devices : 1
Spare
Devices : 1
Rebuild Status : 94% complete
UUID : 63a6d6db:ced50964:3fb69cfe:6b30dd55
Events : 0.4
Number Major Minor
RaidDevice State
0 8 10
0 active sync /dev/sda10
2 8 12
1 spare rebuilding /dev/sda12
3 8 11
- faulty spare /dev/sda11
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