Introduction
This post is about creating a software RAID using the existing partitions. Usually for RAID device we should use different disk. But here since I don’t have that much hardware available, use partitions to demonstrate the software RAID configuration.
There are different level of RAID available. We can create level 0, level 1 and level 5 RAID using software RAID.
Following table gives a brief idea about different RAID level possible by software RAID.
Assuming that each disk is of 10G here we can have
| RAID Level | Min # Disk Required | Effective Storage | Technique |
| Level 0 | 2 | 20G | Stripping |
| Level 1 | 2 | 10G | Mirroring |
| Level 5 | 3 | 20G | Stripping with parity |
| Level 6 | 6 | 20G | Stripping with multiple parity |
Configuring RAID
Following are the simple steps to be done as a system administrator to configure software RAID. I am creating Level 1 RAID here.
Step 1) Creating RAID partitions
RAID partitions are of type fd (Linux RAID Autodetect). Creating 5 partitions of type fd, each of size 1G.
[root@localhost ~]# fdisk /dev/sdb
Command (m for help): p
Disk /dev/sdb: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-652, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-652, default 652): +1G
Command (m for help): t
Selected partition 1
Hex code (type L to list codes): L
0 Empty 1e Hidden W95 FAT1 80 Old Minix be Solaris boot
1 FAT12 24 NEC DOS 81 Minix / old Lin bf Solaris
2 XENIX root 39 Plan 9 82 Linux swap / So c1 DRDOS/sec (FAT-
3 XENIX usr 3c PartitionMagic 83 Linux c4 DRDOS/sec (FAT-
4 FAT16 <32M 40 Venix 80286 84 OS/2 hidden C: c6 DRDOS/sec (FAT-
5 Extended 41 PPC PReP Boot 85 Linux extended c7 Syrinx
6 FAT16 42 SFS 86 NTFS volume set da Non-FS data
7 HPFS/NTFS 4d QNX4.x 87 NTFS volume set db CP/M / CTOS / .
8 AIX 4e QNX4.x 2nd part 88 Linux plaintext de Dell Utility
9 AIX bootable 4f QNX4.x 3rd part 8e Linux LVM df BootIt
a OS/2 Boot Manag 50 OnTrack DM 93 Amoeba e1 DOS access
b W95 FAT32 51 OnTrack DM6 Aux 94 Amoeba BBT e3 DOS R/O
c W95 FAT32 (LBA) 52 CP/M 9f BSD/OS e4 SpeedStor
e W95 FAT16 (LBA) 53 OnTrack DM6 Aux a0 IBM Thinkpad hi eb BeOS fs
f W95 Ext’d (LBA) 54 OnTrackDM6 a5 FreeBSD ee EFI GPT
10 OPUS 55 EZ-Drive a6 OpenBSD ef EFI (FAT-12/16/
11 Hidden FAT12 56 Golden Bow a7 NeXTSTEP f0 Linux/PA-RISC b
12 Compaq diagnost 5c Priam Edisk a8 Darwin UFS f1 SpeedStor
14 Hidden FAT16 ❤ 61 SpeedStor a9 NetBSD f4 SpeedStor
16 Hidden FAT16 63 GNU HURD or Sys ab Darwin boot f2 DOS secondary
17 Hidden HPFS/NTF 64 Novell Netware b7 BSDI fs fd Linux raid auto
18 AST SmartSleep 65 Novell Netware b8 BSDI swap fe LANstep
1b Hidden W95 FAT3 70 DiskSecure Mult bb Boot Wizard hid ff BBT
1c Hidden W95 FAT3 75 PC/IX
Hex code (type L to list codes): fd
Changed system type of partition 1 to fd (Linux raid autodetect)
Command (m for help): p
Disk /dev/sdb: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sdb1 1 123 987966 fd Linux raid autodetect
Like wise create 4 more partition
at the end it should look like as shown below
Command (m for help): p
Disk /dev/sdb: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sdb1 1 123 987966 fd Linux raid autodetect
/dev/sdb2 124 246 987997+ fd Linux raid autodetect
/dev/sdb3 247 369 987997+ fd Linux raid autodetect
/dev/sdb4 370 652 2273197+ 5 Extended
/dev/sdb5 370 492 987966 fd Linux raid autodetect
/dev/sdb6 493 615 987966 fd Linux raid autodetect
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
Step 2) Running partprobe
Running partprobe will ask kernel to read the partition table from disk into memory, so that new partition will be in effect.
[root@localhost ~]# partprobe
Warning: Unable to open /dev/hdc read-write (Read-only file system). /dev/hdc has been opened read-only.
[root@localhost ~]# cat /proc/partitions
major minor #blocks name
8 0 10485760 sda
8 1 104391 sda1
8 2 3068415 sda2
8 3 3068415 sda3
8 4 1 sda4
8 5 1020096 sda5
8 6 1020096 sda6
8 7 514048 sda7
8 8 987966 sda8
8 16 5242880 sdb
8 17 987966 sdb1
8 18 987997 sdb2
8 19 987997 sdb3
8 20 1 sdb4
8 21 987966 sdb5
8 22 987966 sdb6
Step 3) Create a RAID device of defined level.
Here we are using /dev/sdb1 and /dev/sdb2 partition to create Level 1 RAID.
[root@localhost ~]# mdadm -C /dev/md0 -a yes -l 1 -n 2 /dev/sdb{1,2}
mdadm: array /dev/md0 started.
Here mdadm is the command to create raid device
-C is the create option
/dev/md0 is the device name
-a yes option is to create a RAID file if it doesnt exists
-l 1 is the RAID level 1 (mirroring)
-n 2 is number of device 2
/dev/sdb{1,2} are the device names (/dev/sdb1, /dev/sdb2)
Step 4) Format the RAID device
Once the RAID device is created, next step is to format the device. We will format for ext3 type.
[root@localhost ~]# mkfs.ext3 /dev/md0
mke2fs 1.39 (29-May-2006)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
123648 inodes, 246960 blocks
12348 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=255852544
8 block groups
32768 blocks per group, 32768 fragments per group
15456 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 33 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
Step 5) Last step is mounting the file system
[root@localhost ~]# mkdir /raid1
[root@localhost ~]# mount /dev/md0 /raid1
[root@localhost ~]# cd /raid1
[root@localhost raid1]# df -h .
Filesystem Size Used Avail Use% Mounted on
/dev/md0 950M 18M 885M 2% /raid1
As we know that RAID 1 is mirroring, so effective disk space is half that of the provided disk. Here we provided 2 disk partitions of 1G each. So the effective disk space is approx 1G.
You can see the details of RAID device using mdadm –detail command as shown below
[root@localhost raid1]# mdadm –detail /dev/md0
/dev/md0:
Version : 00.90.03
Creation Time : Tue Sep 16 08:59:31 2008
Raid Level : raid1
Array Size : 987840 (964.85 MiB 1011.55 MB)
Device Size : 987840 (964.85 MiB 1011.55 MB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Tue Sep 16 09:02:11 2008
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
UUID : dcf37c14:179f9a7a:ed1f46c6:a8160267
Events : 0.2
Number Major Minor RaidDevice State
0 8 17 0 active sync /dev/sdb1
1 8 18 1 active sync /dev/sdb2
Step 6) Adding a RAID device to /etc/fstab
[root@localhost ~]# cat /etc/fstab | grep md
/dev/md0 /raid1 ext3 defaults 0 0
Next post, we will simulate the failure of one of the RAID disk and try to replace the same.
Hope this helps !!
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