Creating a software raid

Hi
   Today we will look at how we can build a disk array from command line; we can be in a situation where we need to build a software array inorder to have redundancy

We will do it in our ubuntu server, so the first step is to install the mdadm or multidisk administrator


root@ubuntu:~# apt-get install mdadm
Reading package lists... Done
Building dependency tree      
Reading state information... Done
The following packages were automatically installed and are no longer required:
  cyrus-common cyrus-common-2.4 db4.7-util db4.8-util
Use 'apt-get autoremove' to remove them.
The following NEW packages will be installed:
  mdadm
0 upgraded, 1 newly installed, 0 to remove and 2 not upgraded.
Need to get 529 kB of archives.
After this operation, 1,218 kB of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com/ubuntu/ quantal/main mdadm i386 3.2.5-1ubuntu3 [529 kB]
Fetched 529 kB in 1s (386 kB/s)
Preconfiguring packages ...
Selecting previously unselected package mdadm.
(Reading database ... 174173 files and directories currently installed.)
Unpacking mdadm (from .../mdadm_3.2.5-1ubuntu3_i386.deb) ...
Processing triggers for man-db ...
Processing triggers for doc-base ...
Processing 6 added doc-base files...
Processing triggers for ureadahead ...
ureadahead will be reprofiled on next reboot
Setting up mdadm (3.2.5-1ubuntu3) ...
Generating mdadm.conf... done.
 Removing any system startup links for /etc/init.d/mdadm-raid ...
update-initramfs: deferring update (trigger activated)
 * Starting MD monitoring service mdadm --monitor                                                      [ OK ]
Processing triggers for ureadahead ...
Processing triggers for initramfs-tools ...
update-initramfs: Generating /boot/initrd.img-3.5.0-15-generic
W: mdadm: /etc/mdadm/mdadm.conf defines no arrays.


So now we are ready to build our array, in this example I will build a raid1 array that is in other words mirroring


# mdadm --create md0 --raid-devices=2 --level=1 /dev/sdc1 /dev/sdd1
mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
Continue creating array? y      
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md/md0 started.

We invoke mdadm with --create option and then we type the array device in this case I have indicated its name, which is going to be md0, the we specify the number of raid devices that is the number of disks, so for a mirror we can use 2 disks, level=1 means that the array is going to be a mirror, the we specify the disk drives previously formatted.


We may or may not get a warning messages, for me I have used these 2 disks previously. It is going then to ask if it can continue creating the array, we choose y.

If we do fdisk -l we are going to see our new array

#fdisk -l
Disk /dev/md127: 10.7 GB, 10727849984 bytes
2 heads, 4 sectors/track, 2619104 cylinders, total 20952832 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/md127 doesn't contain a valid partition table



we can now partition our new array

fdisk /dev/md127
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel with disk identifier 0x65cfead1.
Changes will remain in memory only, until you decide to write them.
After that, of course, the previous content won't be recoverable.

Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Command (m for help): p

Disk /dev/md127: 10.7 GB, 10727849984 bytes
2 heads, 4 sectors/track, 2619104 cylinders, total 20952832 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x65cfead1

      Device Boot      Start         End      Blocks   Id  System

Command (m for help): n
Partition type:
   p   primary (0 primary, 0 extended, 4 free)
   e   extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-20952831, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-20952831, default 20952831):
Using default value 20952831

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

 0  Empty           24  NEC DOS         81  Minix / old Lin bf  Solaris       
 1  FAT12           27  Hidden NTFS Win 82  Linux swap / So c1  DRDOS/sec (FAT-
 2  XENIX root      39  Plan 9          83  Linux           c4  DRDOS/sec (FAT-
 3  XENIX usr       3c  PartitionMagic  84  OS/2 hidden C:  c6  DRDOS/sec (FAT-
 4  FAT16 <32M      40  Venix 80286     85  Linux extended  c7  Syrinx        
 5  Extended        41  PPC PReP Boot   86  NTFS volume set da  Non-FS data   
 6  FAT16           42  SFS             87  NTFS volume set db  CP/M / CTOS / .
 7  HPFS/NTFS/exFAT 4d  QNX4.x          88  Linux plaintext de  Dell Utility  
 8  AIX             4e  QNX4.x 2nd part 8e  Linux LVM       df  BootIt        
 9  AIX bootable    4f  QNX4.x 3rd part 93  Amoeba          e1  DOS access    
 a  OS/2 Boot Manag 50  OnTrack DM      94  Amoeba BBT      e3  DOS R/O       
 b  W95 FAT32       51  OnTrack DM6 Aux 9f  BSD/OS          e4  SpeedStor     
 c  W95 FAT32 (LBA) 52  CP/M            a0  IBM Thinkpad hi eb  BeOS fs       
 e  W95 FAT16 (LBA) 53  OnTrack DM6 Aux a5  FreeBSD         ee  GPT           
 f  W95 Ext'd (LBA) 54  OnTrackDM6      a6  OpenBSD         ef  EFI (FAT-12/16/
10  OPUS            55  EZ-Drive        a7  NeXTSTEP        f0  Linux/PA-RISC b
11  Hidden FAT12    56  Golden Bow      a8  Darwin UFS      f1  SpeedStor     
12  Compaq diagnost 5c  Priam Edisk     a9  NetBSD          f4  SpeedStor     
14  Hidden FAT16 <3 61  SpeedStor       ab  Darwin boot     f2  DOS secondary 
16  Hidden FAT16    63  GNU HURD or Sys af  HFS / HFS+      fb  VMware VMFS   
17  Hidden HPFS/NTF 64  Novell Netware  b7  BSDI fs         fc  VMware VMKCORE
18  AST SmartSleep  65  Novell Netware  b8  BSDI swap       fd  Linux raid auto
1b  Hidden W95 FAT3 70  DiskSecure Mult bb  Boot Wizard hid fe  LANstep       
1c  Hidden W95 FAT3 75  PC/IX           be  Solaris boot    ff  BBT           
1e  Hidden W95 FAT1 80  Old Minix     
Hex code (type L to list codes): 83

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

Calling ioctl() to re-read partition table.
Syncing disks.








And last but not least we format it and mount it

mkfs -t ext4 /dev/md127p1
mke2fs 1.42.5 (29-Jul-2012)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
655360 inodes, 2618848 blocks
130942 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=2684354560
80 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done                           
Writing inode tables: done                           
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

root@ubuntu:~# mkdir /array
mount /dev/md127p1 /array
root@ubuntu:~# df -kh
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1        31G   11G   18G  38% /
udev            333M  4.0K  333M   1% /dev
tmpfs           137M  1.8M  135M   2% /run
none            5.0M     0  5.0M   0% /run/lock
none            341M   76K  341M   1% /run/shm
none            100M   12K  100M   1% /run/user
/dev/md127p1    9.9G  151M  9.2G   2% /array