Configuring software RAID
This guide will help you configure your server’s RAID array in the event that it needs to be rebuilt due to corruption or disk failure.
Last updated 18th June 2018
Objective
Redundant Array of Independent Disks (RAID) is a utility that mitigates data loss on a server by replicating data across two or more disks.
The default RAID level for OVHcloud server installations is RAID 1, which doubles the space taken up by your data, effectively halving the useable disk space.
This guide will help you configure your server’s RAID array in the event that it needs to be rebuilt due to corruption or disk failure.
Requirements
- a dedicated server with a software RAID configuration
- administrative (root) access to the server via SSH
Instructions
In a command line session, type the following code to determine the current RAID status:
cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty]
md2 : active raid1 sdb2[1] sda2[0] sdc2[2]
96211904 blocks [3/3] [UUU]
md1 : active raid1 sdc1[2] sdb1[1] sda1[0]
20478912 blocks [3/3] [UUU]
unused devices: <none>
This command shows us that we have two RAID arrays currently set up, with md2 being the largest partition. The partition consists of all three disks, which are known as sda2, sdb2 and sdc2. The [UUU] means that all the disks are working normally. A _ would indicate a failed disk.
Although this command returns our RAID volumes, it doesn't tell us the size of the partitions themselves. We can find this information with the following command:
fdisk -l
Disk /dev/sda: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders, total 234441648 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: 0x000e9ae1
Device Boot Start End Blocks Id System
/dev/sda1 * 4096 40962047 20478976 fd Linux RAID autodetect
/dev/sda2 40962048 233385983 96211968 fd Linux RAID autodetect
/dev/sda3 233385984 234432511 523264 82 Linux swap / Solaris
Disk /dev/sdb: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders, total 234441648 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: 0x00001af8
Device Boot Start End Blocks Id System
/dev/sdb1 * 4096 40962047 20478976 fd Linux RAID autodetect
/dev/sdb2 40962048 233385983 96211968 fd Linux RAID autodetect
/dev/sdb3 233385984 234432511 523264 82 Linux swap / Solaris
Disk /dev/sdc: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders, total 234441648 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: 0x0000da80
Device Boot Start End Blocks Id System
/dev/sdc1 * 4096 40962047 20478976 fd Linux RAID autodetect
/dev/sdc2 40962048 233385983 96211968 fd Linux RAID autodetect
/dev/sdc3 233385984 234432511 523264 82 Linux swap / Solaris
Disk /dev/md1: 21.0 GB, 20970405888 bytes
2 heads, 4 sectors/track, 5119728 cylinders, total 40957824 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/md1 doesn't contain a valid partition table
Disk /dev/md2: 98.5 GB, 98520989696 bytes
2 heads, 4 sectors/track, 24052976 cylinders, total 192423808 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/md2 doesn't contain a valid partition table
We can see that /dev/md1 consists of 21GB and /dev/md2 contains 98.5GB. If we were to run the mount command we can also find out the layout of the disk.
/dev/root on / type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
devtmpfs on /dev type devtmpfs (rw,relatime,size=16417956k,nr_inodes=4104489,mode=755)
sysfs on /sys type sysfs (rw,relatime)
none on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
none on /proc type proc (rw,nosuid,nodev,noexec,relatime)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,nosuid,nodev,noexec,relatime)
none on /sys/fs/cgroup type tmpfs (rw,relatime,size=4k,mode=755)
none on /sys/fs/fuse/connections type fusectl (rw,relatime)
none on /sys/kernel/security type securityfs (rw,relatime)
none on /run type tmpfs (rw,nosuid,noexec,relatime,size=3283700k,mode=755)
none on /sys/fs/pstore type pstore (rw,relatime)
none on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
none on /run/shm type tmpfs (rw,nosuid,nodev,relatime)
none on /run/user type tmpfs (rw,nosuid,nodev,noexec,relatime,size=102400k,mode=755)
systemd on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,name=systemd)
gvfsd-fuse on /run/user/109/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=109,group_id=117)
gvfsd-fuse on /run/user/0/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0)
/dev/md2 on /home type ext4 (rw,relatime,data=ordered)
As the disks are currently mounted by default, to remove a disk from the RAID, we first need to unmount the disk, then simulate a failure, and finally remove it. We will remove /dev/sda2 from the RAID with the following command:
umount /dev/md2
This will provide us with the following output:
/dev/root on / type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
devtmpfs on /dev type devtmpfs (rw,relatime,size=16417956k,nr_inodes=4104489,mode=755)
sysfs on /sys type sysfs (rw,relatime)
none on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
none on /proc type proc (rw,nosuid,nodev,noexec,relatime)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,nosuid,nodev,noexec,relatime)
none on /sys/fs/cgroup type tmpfs (rw,relatime,size=4k,mode=755)
none on /sys/fs/fuse/connections type fusectl (rw,relatime)
none on /sys/kernel/security type securityfs (rw,relatime)
none on /run type tmpfs (rw,nosuid,noexec,relatime,size=3283700k,mode=755)
none on /sys/fs/pstore type pstore (rw,relatime)
none on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
none on /run/shm type tmpfs (rw,nosuid,nodev,relatime)
none on /run/user type tmpfs (rw,nosuid,nodev,noexec,relatime,size=102400k,mode=755)
systemd on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,name=systemd)
gvfsd-fuse on /run/user/109/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=109,group_id=117)
gvfsd-fuse on /run/user/0/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0)
As we can see the, entry of /dev/md2 is no longer mounted. However, the RAID is still active, so we need to simulate a failure to remove the disk. We can do this with the following command:
mdadm --fail /dev/md2 /dev/sda2
We have now simulated a failure of the RAID. The next step is to remove the partition from the RAID array with the following command:
mdadm --remove /dev/md2 /dev/sda2
You can verify that the partition has been removed with the following command:
cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty]
md2 : active raid1 sda2[0] sdb2[1] sdc2[2]
96211904 blocks [3/2] [_UU]
md1 : active raid1 sdc1[2] sdb1[1] sda1[0]
20478912 blocks [3/3] [UUU]
unused devices: <none>
The following command will verify that the partition has been removed:
mdadm --detail /dev/md2
/dev/md2:
Version : 0.90
Creation Time : Wed Apr 29 16:58:08 2015
Raid Level : raid1
Array Size : 96211904 (91.75 GiB 98.52 GB)
Used Dev Size : 96211904 (91.75 GiB 98.52 GB)
Raid Devices : 3
Total Devices : 2
Preferred Minor : 2
Persistence : Superblock is persistent
Update Time : Sat Jun 20 12:55:24 2015
State : clean, degraded
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
UUID : 7f39d062:9f16a016:a4d2adc2:26fd5302
Events : 0.90
Number Major Minor RaidDevice State
0 0 0 0 removed
1 8 18 1 active sync /dev/sdb2
2 8 34 2 active sync /dev/sdc2
Once the disk has been replaced, we need to copy the partition table from a healthy disk (in this example, sdb) to the new one (sda) with the following command:
sfdisk -d /dev/sdb | sfdisk /dev/sda
We can now rebuild the RAID array. The following code snippet shows how we can rebulid the /dev/md2 partition layout with the recently-copied sda partition table:
mdadm --add /dev/md2 /dev/sda2
cat /proc/mdstat
We can verify the RAID details with the following command:
mdadm --detail /dev/md2
/dev/md2:
Version : 0.90
Creation Time : Mon Mar 05 16:58:08 2018
Raid Level : raid1
Array Size : 96211904 (91.75 GiB 98.52 GB)
Used Dev Size : 96211904 (91.75 GiB 98.52 GB)
Raid Devices : 3
Total Devices : 3
Preferred Minor : 2
Persistence : Superblock is persistent
Update Time : Tue Mar 06 16:58:08 2018
State : clean, degraded, recovering
Active Devices : 2
Working Devices : 3
Failed Devices : 0
Spare Devices : 1
Rebuild Status : 21% complete
UUID : 7f39d062:9f16a016:a4d2adc2:26fd5302
Events : 0.95
Number Major Minor RaidDevice State
3 8 2 0 spare rebuilding /dev/sda2
1 8 18 1 active sync /dev/sdb2
2 8 34 2 active sync /dev/sdc2
The RAID has now been rebuilt, but we still need to mount the partition (/dev/md2 in this example) with the following command:
mount /dev/md2 /home
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