Wednesday, 2 May 2012

RAID in RHEL 6


                  Redundant Array of Independent Disks, originally Redundant Array of Inexpensive Disks is a storage technology that combines multiple disk drive components into a logical unit.

Disk arrays stripe data across multiple disks and access them in parallel to achieve:

Higher data transfer rates on large data accesses and
Higher I/O rates on small data accesses.

Level     Description

RAID 0 Block-level striping without parity or mirroring.

RAID 1 Mirroring without parity or striping.

RAID 2 Bit-level striping with dedicated Hamming-code parity.

RAID 3    Byte-level striping with dedicated parity.

RAID 4 Block-level striping with dedicated parity.

RAID 5 Block-level striping with distributed parity.

RAID 6 Block-level striping with double distributed parity.

RAID 10 Nested RAIDs or hybrid RAIDs are constructed by distributing data over mirrored sets of storage devices.


Mirroring plus striping (striped-mirror, RAID-1+0 or RAID-10)

            The combination of striping above mirroring is called a striped-mirror layout. Putting mirroring below striping mirrors each column of the stripe. If there are multiple sub disks per column, each subdisk can be mirrored individually instead of each column. A striped-mirror volume is an example of a layered volume. 

Nesting


Level Description Minimum # of disks
 
RAID 0+1     Top Level RAID 1, Bottom Level RAID 0.    3
RAID 1+0     Top Level RAID 0, Bottom Level RAID 1. 4
RAID 5+0 Top Level RAID 0, Bottom Level RAID 5.    6
RAID 5+1 Top Level RAID 1, Bottom Level RAID 5. 6
RAID 6+0     Top Level RAID 0, Bottom Level RAID 6.    8
RAID 6+1     Top Level RAID 1, Bottom Level RAID 6.    8
RAID 1+0+0   RAID 0 RAID 0 RAID 1 8


Software RAID - the RAID task runs on the CPU of your computer system.

Hardware RAID - solution has its own processor and memory to run the RAID application.


RAID in RHEL 6

The minimum number of software RAID partitions required for each RAID level:
        • RAID   0,1,10   – 2 partitions
        • RAID   4,5        – 3 partitions
        • RAID   6           – 4 partitions

#create partitions
fdisk /dev/sdb
Make 4 partitions of each 1GB
Change System ID as fd to Linux raid autodetect

#create raid
mdadm -C /dev/md0 -l 5 -n 3 /dev/sdb1 /dev/sdb2 /dev/sdb3
C-create
5-raid level
3-raid devices

#mount
mkfs.ext4 /dev/md0
mkdir /raid5

vim /etc/fstab
/dev/md0 /raid5 ext4 defaults 0 0

cd /raid5/
ls

#raid status
mdadm --detail /dev/md0
cat /proc/mkstat

#set faulty partition
mdadm -f /dev/md0 /dev/sdb3
mdadm: set /dev/sdb3 faulty in /dev/md0

#add new raid partition
mdadm -a /dev/md0 /dev/sdb4
mdadm: added /dev/sdb4

#remove faulty raid partition
mdadm -r /dev/md0 /dev/sdb3
mdadm: hot removed /dev/sdb3 from /dev/md0


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