RAID - redundant array of inexpensive disks

RAID basics

most common:  RAID 0, 1, 5 and in some cases 1+0 (10) 

RAID 0 - "disk striping."  - written to more than two disks - faster writes/reads - NO fault tolerance

RAID 1 - "disk mirroring."  - duplicates drive, cuts capacity in have - excellent fault tolerance.

RAID 5 - distributes data across 3 or more disks with parity bits.  hot swappable.  slower writes, faster reads - very common

RAID 6  - like RAID 5 but uses one more parity block, two disks can fail, slower writes, faster reads - not as common

RAID 10 or RAID 1+0  - combination of RAID 1 and 0.  stripe and mirror - fast and fault tolerant - twice as many disks needed

RAID 2 is similar to RAID 5, striping occurs at the bit-level. expensive (requires 10 disks or more) and gives poor performance with some disk I/O operations.
RAID 3 is similar to RAID 5, has a dedicated parity drive. used for specialized applications - rarely used.
RAID 4 disk striping at the byte level, rather than at the bit-level as in RAID 3. - rarely used.
RAID 7 is a proprietary level of RAID owned by the now-defunct Storage Computer Corporation.
RAID 0+1 is a mirrored array with segments that are RAID 0 arrays. 

1. Create a new RAID array

mdadm --create is used to create a new array: --> mdadm --create --verbose /dev/md0 --level=1 /dev/sda1 /dev/sdb2 compact notation: --> mdadm -Cv /dev/md0 -l1 -n2 /dev/sd[ab]1

2. /etc/mdadm.conf - main configuration file

/etc/mdadm.conf or /etc/mdadm/mdadm.conf (on debian) UPDATE file after RAID creation by: --> mdadm --detail --scan >> /etc/mdadm.conf Debian: mdadm --detail --scan >> /etc/mdadm/mdadm.conf

3. Remove a disk from an array

A drive must be failed before it can be removed from the array. It can be in a failed state because of a fault - remove it. Or Fail it using: --> mdadm --fail /dev/md0 /dev/sda1 then remove it: --> mdadm --remove /dev/md0 /dev/sda1 Single command using: --> mdadm /dev/md0 --fail /dev/sda1 --remove /dev/sda1

4. Add a disk to an array

add or replace a new disk to an array --> mdadm --add /dev/md0 /dev/sdb1

5. Status of the RAID arrays

check the status:  --> cat /proc/mdstat  or,

		   -->  mdadm --detail /dev/md0

The output of this command will look like:

--> cat /proc/mdstat
      Personalities : [raid1]
      md0 : active raid1 sdb1[1] sda1[0]
      104320 blocks [2/2] [UU]
      md1 : active raid1 sdb3[1] sda3[0]
      19542976 blocks [2/2] [UU]
      md2 : active raid1 sdb4[1] sda4[0]
      223504192 blocks [2/2] [UU]

both drives are in use and working – 

Failed drive shows as F 
degraded array will miss the 2nd disk 

Monitor the status of a RAID rebuild using watch:

	watch cat /proc/mdstat

6. Stop and delete a RAID array

To completely remove a raid array we stop if first, then remove it: --> mdadm --stop /dev/md0 --> mdadm --remove /dev/md0 and then we can delete the superblock from the individual drives: --> mdadm --zero-superblock /dev/sda In using RAID 1 arrays, we create identical partitions on both drives so we can copy the partitions from sda to sdb by using sfdisk: --> sfdisk -d /dev/sda | sfdisk /dev/sdb This "dumps" the partition table of sda on sdb, removing existing partitions on sdb... without warning.

mdadm help info

to install: (SuSE has it installed by default, if not: zypper install mdadm) # yum install mdadm [on RedHat systems] # apt-get install mdadm [on Debain systems] --> man mdadm | col -b | grep -v ^$ | tee -a man.mdadm.txt --> mdadm --help see also:

RAID 0 setup using mdadm - 2 disks

RAID-0: --> mdadm --create --verbose /dev/md0 --level=stripe --raid-devices=2 /dev/sdb6 /dev/sdc5

RAID 1 setup using mdadm - 2 disks

RAID-1: --> mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdb1 /dev/sdc1

RAID 1 setup using mdadm - 2 disks with a spare

Add a spare disk: --> mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdb1 /dev/sdc1 --spare-devices=1 /dev/sdd1 Check the /proc/mdstat file to see the status of the mirror.

RAID 5 setup using mdadm - 3 disks with a spare

--> mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1 --spare-devices=1 /dev/sde1

saving RAID configuration

MUD: (MUD = Mint, Ubuntu, or Debian) mdadm --detail --scan >> /etc/mdadm/mdadm.conf Others (check your distribution's documentation): mdadm --detail --scan >> /etc/mdadm.conf
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