Software RAID 1 Configuration in Linux

From the different level's of raid available, raid 1 is better known for redundancy without stripping. This level of raid employs mirroring, by completely replicating the entire data block by block on the one disk to the other.

The minimum number of disk required to create a raid 1 array is 2. Maximum disk number can go up to 32. Number of disk's used must always be an even number.

Different types of raid and its internal working is explained in the below post, along with a configuration post on raid 0 in linux.

Read: Raid Working: A complete Tutorial

Read: How to configure Software Raid 0 in Linux

Read:How to configure software Raid5

As per raid 1 requirement we need minimum two partition.So we create two partition /dev/sda8 and /dev/sda9 and change type of partition to raid type.

[root@localhost ~]# fdisk /dev/sda

The number of cylinders for this disk is set to 19457.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)

Command (m for help): n
First cylinder (18895-19457, default 18895):
Using default value 18895
Last cylinder or +size or +sizeM or +sizeK (18895-19457, default 19457): +100M

Command (m for help): n
First cylinder (18908-19457, default 18908):
Using default value 18908
Last cylinder or +size or +sizeM or +sizeK (18908-19457, default 19457): +100M

Command (m for help): t
Partition number (1-9): 9
Hex code (type L to list codes): fd
Changed system type of partition 9 to fd (Linux raid autodetect)

Command (m for help): t
Partition number (1-9): 8
Hex code (type L to list codes): fd
Changed system type of partition 8 to fd (Linux raid autodetect)

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table.
The new table will be used at the next reboot.
Syncing disks.
[root@localhost ~]#

Explanation:

fdisk:           tool to create new partition

n    :            means create a new partition

8,9 :             are partition numbers

+100M:        size of partition

t     :            means change the type of mentioned partition

fd:               code for raid type partition

w:                is to save the change in table

Use partprobe command to update the Partition Table without Reboot and then view the table.

[root@localhost ~]# partprobe
[root@localhost ~]# fdisk -l

Disk /dev/sda: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13      102400    7  HPFS/NTFS
Partition 1 does not end on cylinder boundary.
/dev/sda2              13        3825    30617600    7  HPFS/NTFS
/dev/sda3            3825       11474    61440000    7  HPFS/NTFS
/dev/sda4           11475       19457    64123447+   5  Extended
/dev/sda5           11475       18868    59392273+  83  Linux
/dev/sda6           18869       18881      104391   fd  Linux raid autodetect
/dev/sda7           18882       18894      104391   fd  Linux raid autodetect
/dev/sda8           18895       18907      104391   fd  Linux raid autodetect
/dev/sda9           18908       18920      104391   fd  Linux raid autodetect

Disk /dev/md0: 213 MB, 213647360 bytes
2 heads, 4 sectors/track, 52160 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md0 doesn't contain a valid partition table

Now Create RAID 1 Device called it /dev/md1:

[root@localhost ~]# mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sda8 /dev/sda9

Now to see the information about created raid device /dev/md1 in detail use following command:

[root@localhost ~]# mdadm --detail /dev/md1
/dev/md1:
        Version : 0.90
  Creation Time : Tue Apr  9 14:50:00 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 : 1
    Persistence : Superblock is persistent

    Update Time : Tue Apr  9 14:50:07 2013
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 034c85f3:60ce1191:3b61e8dc:55851162
         Events : 0.2

    Number   Major   Minor   RaidDevice State
       0       8        8        0      active sync   /dev/sda8
       1       8        9        1      active sync   /dev/sda9

[root@localhost ~]# cat /proc/mdstat
Personalities : [raid0] [raid1]
md1 : active raid1 sda9[1] sda8[0]
      104320 blocks [2/2] [UU]
      
md0 : active raid0 sda7[1] sda6[0]
      208640 blocks 64k chunks
      
unused devices: <none>

From above output one can clearly understand that raid1 have been created and using partitions /dev/sda8 and /dev/sda9.
 

FORMAT RAID DEVICE /dev/md1:

[root@localhost ~]# mke2fs -j /dev/md1
mke2fs 1.39 (29-May-2006)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
26104 inodes, 104320 blocks
5216 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
13 block groups
8192 blocks per group, 8192 fragments per group
2008 inodes per group
Superblock backups stored on blocks:
        8193, 24577, 40961, 57345, 73729

Writing inode tables: done                            
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 20 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

Now create a directory /raid1 and mount /dev/md1 on it.

[root@localhost ~]# mkdir /raid1
[root@localhost ~]# mount /dev/md1 /raid1
[root@localhost ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda5              55G   18G   35G  34% /
tmpfs                 502M     0  502M   0% /dev/shm
/dev/sda3              59G   31G   28G  53% /root/Desktop/win7
/dev/md0              198M  5.8M  182M   4% /raid0
/dev/md1               99M  5.6M   89M   6% /raid1
[root@localhost ~]

Above output clearly show that raid1 is successfully created and mounted on /raid1.

HARD DISK FAILURE & RECOVERY TEST

Now what will happen with the data on /raid1 directory, if any of disk fail or crash?

Will raid1 protect the data from such loss?

To get the answer of the above question we will make some files on /raid1 directory. After that we manually delete or remove any one hard disk from raid1 devices and then check the contents status.

[root@localhost ~]# cd /raid1

[root@localhost raid1]# touch {a..z}
[root@localhost raid1]# ls
a  b  c  d  e  f  g  h  i  j  k  l  lost+found  m  n  o  p  q  r  s  t  u  v  w  x  y  z
[root@localhost raid1]#

Now we are going to fail /dev/sda9 partition from raid1 and then check the status of data.

[root@localhost ~]# mdadm /dev/md1 --fail /dev/sda9
mdadm: set /dev/sda9 faulty in /dev/md1
[root@localhost ~]# mdadm --detail /dev/md1
/dev/md1:
        Version : 0.90
  Creation Time : Tue Apr  9 14:50:00 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 : 1
    Persistence : Superblock is persistent

    Update Time : Tue Apr  9 15:47:22 2013
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0

           UUID : 034c85f3:60ce1191:3b61e8dc:55851162
         Events : 0.4

    Number   Major   Minor   RaidDevice State
       0       8        8        0      active sync   /dev/sda8
       1       0        0        1      removed

       2       8        9        -      faulty spare   /dev/sda9

[root@localhost ~]# ls -l /raid1
total 12
-rw-r--r-- 1 root root     0 Apr  9 15:43 a
-rw-r--r-- 1 root root     0 Apr  9 15:43 b
-rw-r--r-- 1 root root     0 Apr  9 15:43 c
-rw-r--r-- 1 root root     0 Apr  9 15:43 d
-rw-r--r-- 1 root root     0 Apr  9 15:43 e
-rw-r--r-- 1 root root     0 Apr  9 15:43 f
-rw-r--r-- 1 root root     0 Apr  9 15:43 g
-rw-r--r-- 1 root root     0 Apr  9 15:43 h
-rw-r--r-- 1 root root     0 Apr  9 15:43 i
-rw-r--r-- 1 root root     0 Apr  9 15:43 j
-rw-r--r-- 1 root root     0 Apr  9 15:43 k
-rw-r--r-- 1 root root     0 Apr  9 15:43 l
drwx------ 2 root root 12288 Apr  9 15:21 lost+found
-rw-r--r-- 1 root root     0 Apr  9 15:43 m
-rw-r--r-- 1 root root     0 Apr  9 15:43 n
-rw-r--r-- 1 root root     0 Apr  9 15:43 o
-rw-r--r-- 1 root root     0 Apr  9 15:43 p
-rw-r--r-- 1 root root     0 Apr  9 15:43 q
-rw-r--r-- 1 root root     0 Apr  9 15:43 r
-rw-r--r-- 1 root root     0 Apr  9 15:43 s
-rw-r--r-- 1 root root     0 Apr  9 15:43 t
-rw-r--r-- 1 root root     0 Apr  9 15:43 u
-rw-r--r-- 1 root root     0 Apr  9 15:43 v
-rw-r--r-- 1 root root     0 Apr  9 15:43 w
-rw-r--r-- 1 root root     0 Apr  9 15:43 x
-rw-r--r-- 1 root root     0 Apr  9 15:43 y
-rw-r--r-- 1 root root     0 Apr  9 15:43 z

Hence we can clearly see that even after the removal of one partition(/dev/sda9) our data is still safe because of mirroring.
 

Now what if you  want to add a new partition or hard disk in place of faulty spare?

INSERTING A NEW HARD DISK:

• first create a new partition.
• change type of partition to raid type
• Then add the created partition to existing raid.

Follow the exact same steps shown in the beginning of this tutorial to create partition and modifying its type to RAID. Suppose the newly created partition is /dev/sda10

Now add this newly created partition to existing raid using mdadm command:

[root@localhost ~]# mdadm /dev/md1 --add /dev/sda10
mdadm: added /dev/sda10

Hence newly created partition with raid type have been successfully addes to raid1.

[root@localhost ~]# cat /proc/mdstat
Personalities : [raid0] [raid1]
md1 : active raid1 sda10[1] sda9[2](F) sda8[0]
      104320 blocks [2/2] [UU]
      
md0 : active raid0 sda7[1] sda6[0]
      208640 blocks 64k chunks
      
unused devices: <none>
[root@localhost ~]#

KNOW THE ADVANTAGES OF RAID 1:

• When you go through this article you will observe that the RAID 1 technology is very simple.
• The most important advantages of using RAID 1 is that in case of a disk failure,data do not have to be rebuild.They just copied to the replacement disk.
• Read & Write speed is excellent in comparison to that of a single disk.

LEARN THE DISADVANTAGES OF RAID 1:

• The most worst disadvantages of RAID1 is that it can only uses 50% of Storage Device because every data has to be written twice.