How To do Software RAID using mdadm in CentOS

Today I am writing about how to create Software RAID. Creating Software RAID is pretty easy. All we need either 3 equal size partitions on different harddisks or 3 equal size harddisks.

        In my approach , I am doing this in VmWare by choosing 3 virtual harddisks with equal 1 GB             Size. Our first step is we need to make those harddisks as ready for RAID.

        In your terminal type as

          # fdisk -l
       To see available partitions and then try to create RAID partitions in that.

        If We assume my 3 harddisks as /dev/sdb,/dev/sdc,/dev/sdd then in your terminal

          # fdisk /dev/sdb

        Then
       
         [root@ns1 sysadmin]# fdisk /dev/sdb

WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').
Command (m for help): m
Command action
   a   toggle a bootable flag
   b   edit bsd disklabel
   c   toggle the dos compatibility flag
   d   delete a partition
   l   list known partition types
   m   print this menu
   n   add a new partition
   o   create a new empty DOS partition table
   p   print the partition table
   q   quit without saving changes
   s   create a new empty Sun disklabel
   t   change a partition's system id
   u   change display/entry units
   v   verify the partition table
   w   write table to disk and exit
   x   extra functionality (experts only)
Command (m for help): n

          I am selecting new partition here as primary with default size ,( all my 3 VHDS having same size)

Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-130, default 1): 
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-130, default 130): 
Using default value 130

Now we have to change Partition type to RAID , so

Command (m for help): t
Selected partition 1
Hex code (type L to list codes):  fd ( fd for auto RAID)

Now just verify

Command (m for help): p
Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x8a6b2eca
   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1         130     1044193+  fd  Linux raid autodetect

All good , we can commit our changes.

Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.

Same as above do the same for /dev/sdc and /dev/sdd and then finally it will be like

[root@ns1 sysadmin]# fdisk -l
Disk /dev/sda: 21.5 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00092c9c
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          64      512000   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              64        2611    20458496   8e  Linux LVM
Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x8a6b2eca
   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1         130     1044193+  fd  Linux raid autodetect
Disk /dev/sdc: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x6429c3f4
   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               1         130     1044193+  fd  Linux raid autodetect
Disk /dev/sdd: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xe883f58a
   Device Boot      Start         End      Blocks   Id  System
/dev/sdd1               1         130     1044193+  fd  Linux raid autodetect
Disk /dev/mapper/VolGroup-lv_root: 18.9 GB, 18865979392 bytes
255 heads, 63 sectors/track, 2293 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/mapper/VolGroup-lv_swap: 2080 MB, 2080374784 bytes
255 heads, 63 sectors/track, 252 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Now we have to create RAID, Here I am choosing RAID 5 which needs minimum 3 RAID Harddisks

[root@ns1 sysadmin]# mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

RAID 5 Created. --level indicates RAID level and after number and name of the devices.

To see your RAID Information execute this command

[root@ns1 sysadmin]# mdadm --detail /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Tue Nov 11 00:05:20 2014
     Raid Level : raid5
     Array Size : 2086912 (2038.34 MiB 2137.00 MB)
  Used Dev Size : 1043456 (1019.17 MiB 1068.50 MB)
   Raid Devices : 3
  Total Devices : 3
    Persistence : Superblock is persistent
    Update Time : Tue Nov 11 00:05:26 2014
          State : clean
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0
         Layout : left-symmetric
     Chunk Size : 512K
           Name : ns1.example.com:0  (local to host ns1.example.com)
           UUID : af2998ef:acc6cb1e:feb832f1:7f9b2f1b
         Events : 18
    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       1       8       33        1      active sync   /dev/sdc1
       3       8       49        2      active sync   /dev/sdd1

Then we have to save our RAID Configuration.

In CentOS configuration file : /etc/mdadm.conf
In Debain baed /etc/mdadm/mdadm.conf

So save the configuration with

[root@ns1 sysadmin]# mdadm --detail --scan
ARRAY /dev/md0 metadata=1.2 name=ns1.example.com:0 UUID=af2998ef:acc6cb1e:feb832f1:7f9b2f1b
[root@ns1 sysadmin]# mdadm --detail --scan > /etc/mdadm.conf
[root@ns1 sysadmin]# 

if you want to see stats of your RAID

[root@ns1 sysadmin]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] 
md0 : active raid5 sdd1[3] sdc1[1] sdb1[0]
      2086912 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]

To set any RAID Disk as failed/faulty

# mdadm --fail /dev/md0 /dev/sdb1

To Remove any Disk from RAID

# mdamd --remove /dev/md0 /dev/sdb1

To add new disk

# mdadm --add /dev/md0 /dev/sdb1

If harddisk/partition has taken from another RAID , to flush all data in it before using into a new RAID

# mdadm --zero-superblock  /dev/sda1

Stop a Running RAID

# mdadm --stop /dev/md0

and to start

# mdadm --run /dev/md0 

Its supposed to run but with research i have done it wont run/start.

To start it we have to do like assembling again

# mdadm --assemble /dev/md0 /dev/sda1 /dev/sdb1 /dev/sdc1 

If you UUID of /dev/md0 it will be more easier and perfect.

# mdadm --assemble --scan --uuid=af2998ef:acc6cb1e:feb832f1:7f9b2f1b


Source:

http://www.devil-linux.org/documentation/1.0.x/ch01s05.html
https://raid.wiki.kernel.org/index.php/RAID_setup