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HOWTO: Create a new RAID 1 (mirror)
27/12/08
01:11:41 pm, by richard 
, 786 words 
Categories: Ubuntu Linux, OS and Systems Administration, Filing Systems, Installation and Configuration
HOWTO: Create a new RAID 1 (mirror)
You have two devices of approximately same size and want to mirror the data on one of them to the other to make your system more secure.
The number of cylinders for this disk is set to 30401.
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):
Use the p command to see the current partitions on the drive or just to make sure you have the right one 
Command (m for help): p
Disk /dev/sdb: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00045339
Device Boot Start End Blocks Id System
Add a new partition making sure the same size will fit on both disks if they are not exactly the same.
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-30401, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-30401, default 30401):
Using default value 30401
Lastly set the dick ID type to Linux RAID. This is what it would be set to if you have created the RAID 1 array while installing Ubuntu. I'm not going to argue with the guys that produce this great Linux distribution.
Command (m for help): t
Selected partition 1
Hex code (type L to list codes): fd
Changed system type of partition 1 to fd (Linux RAID autodetect)
Just one more check that all is as expected before writing the new values to the disk.
Command (m for help): p
Disk /dev/sdb: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00045339
Device Boot Start End Blocks Id System
/dev/sdb1 1 30401 244196001 fd Linux RAID autodetect
This looks good to me so write away!
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
Do the same again for he second disk and we will be ready to create the new mirrored disk array.
mdadm: size set to 244195904K
mdadm: array /dev/md1 started.
Note: I used md1 as the device name as I already used md0 for the raid 5 array.
You can now watch the array sync itself over the two disks.
watch -n 7 cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md1 : active raid1 sdc1[1] sdb1[0]
244195904 blocks [2/2] [UU]
[>....................] resync = 1.7% (4328640/244195904) finish=48.5min speed=82380K/sec
unused devices: <none>
You will of course need to format it, I use reiserfs so this is done with the following command. I like to label my disks as I think its make life a little simpler.
Guessing about desired format.. Kernel 2.6.24-22-generic is running.
Format 3.6 with standard journal
Count of blocks on the device: 61048976
Number of blocks consumed by mkreiserfs formatting process: 10075
Blocksize: 4096
Hash function used to sort names: "r5"
Journal Size 8193 blocks (first block 18)
Journal Max transaction length 1024
inode generation number: 0
UUID: 1ab2c3d4-51b6-4471-b356-906cb9281293
LABEL: mirror_root
ATTENTION: YOU SHOULD REBOOT AFTER FDISK!
ALL DATA WILL BE LOST ON '/dev/md1'!
Continue (y/n):y
Initializing journal - 0%....20%....40%....60%....80%....100%
Syncing..ok
Tell your friends to use a kernel based on 2.4.18 or later, and especially not a
kernel based on 2.4.9, when you use reiserFS. Have fun.
ReiserFS is successfully created on /dev/md1.
You can now mount the drive from the command line using
sudo mkdir /mnt/mirror
sudo mount /dev/md1 /mnt/mirror
or from within the fstab file with any ONE of the following. The UUID can be found by looking at the message given out from the command above when you put reiserfs on the array.
LABEL=mirror_root /mnt/mirror reiserfs relatime 0 2
/dev/md1 /mnt/mirror reiserfs relatime 0 2
UUID=1ab2c3d4-51b6-4471-b356-906cb9281293 /mnt/mirror reiserfs relatime 0 2
The same method can be used to create RAID5 or RAID0 arrays. take a look at he RAID Setup pages here.
Finally you will need to update mdadm.conf with the new drive adding it to the existing list. Too find out all the UUIDs on the system you can use
/dev/sda1: UUID="a0b4a885-6d24-48c8-b7fc-0000395781f9" TYPE="reiserfs"
/dev/sdb1: UUID=1ab2c3d4-51b6-4471-b356-906cb9281293" TYPE="linux_raid_member"
/dev/sdc1: UUID="89c286e1-79cc-bbfa-d62d-508b2fbf3d7b" TYPE="linux_raid_member"
Edit /etc/mdadm/mdadm.conf and add the new drive to the ARRAY that of disk you have added the new drive to. Do not just copy my UUIDs they will not be the same as yours 
D
Now you need to rebuild the RAM disk image initrd.
Take a look at the man pages for mdadm.conf and update-initramfs.
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