Chris Tyler - Fedora Linux

# mkdir /mnt/mysql

# mount /dev/test/mysql /mnt/mysql

You can simulate the failure of a RAID array element using mdadm :

# mdadm --fail /dev/md0 /dev/sdc1

mdadm: set /dev/sdc1 faulty in /dev/md0

The "failed" drive is marked with the symbol (F) in /proc/ mdstat :

# cat /proc/mdstat

Personalities : [raid1]

md0 : active raid1 sdc1[2](F) sdb1[0]

63872 blocks [2/1] [U_]

unused devices:

To place the "failed" element back into the array, remove it and add it again:

# mdadm --remove /dev/md0 /dev/sdc1

mdadm: hot removed /dev/sdc1

# mdadm --add /dev/md0 /dev/sdc1

mdadm: re-added /dev/sdc1

# cat /proc/mdstat

Personalities : [raid1]

md0 : active raid1 sdc1[1] sdb1[0]

63872 blocks [2/1] [U_]

[>....................] recovery = 0.0% (928/63872) finish=3.1min speed=309K/sec

unused devices:

If the drive had really failed (instead of being subject to a simulated failure), you would replace the drive after removing it from the array and before adding the new one.

Do not hot-plug disk drivesi.e., physically remove or add them with the power turned onunless the drive, disk controller, and connectors are all designed for this operation. If in doubt, shut down the system, switch the drives while the system is turned off, and then turn the power back on.

If you check /proc/mdstat a short while after readding the drive to the array, you can see that the RAID system automatically rebuilds the array by copying data from the good drive(s) to the new drive:

# cat /proc/mdstat

Personalities : [raid1]

md0 : active raid1 sdc1[1] sdb1[0]

63872 blocks [2/1] [U_]

[=============>.......] recovery = 65.0% (42496/63872)

finish=0.8min speed=401K/sec

unused devices:

The mdadm command shows similar information in a more verbose form:

# mdadm -D /dev/md0

/dev/md0:

Version : 00.90.03

Creation Time : Thu Mar 30 01:01:00 2006

Raid Level : raid1

Array Size : 63872 (62.39 MiB 65.40 MB)

Device Size : 63872 (62.39 MiB 65.40 MB)

Raid Devices : 2

Total Devices : 2

Preferred Minor : 0

Persistence : Superblock is persistent

Update Time : Thu Mar 30 01:48:39 2006

State : clean, degraded, recovering

Active Devices : 1

Working Devices : 2

Failed Devices : 0

Spare Devices : 1

Rebuild Status : 65% complete

UUID : b7572e60:4389f5dd:ce231ede:458a4f79

Events : 0.34

Number Major Minor RaidDevice State

0 8 17 0 active sync /dev/sdb1

1 8 33 1 spare rebuilding /dev/sdc1

A RAID array can be stopped anytime that it is not in useuseful if you have built an array incorporating removable or external drives that you want to disconnect. If you're using the RAID device as an LVM physical volume, you'll need to deactivate the volume group so the device is no longer considered to be in use:

# vgchange test -an

0 logical volume(s) in volume group "test" now active

The -an argument here means activated: no . (Alternately, you can remove the PV from the VG using vgreduce .)

To stop the array, use the --stop option to mdadm :

# mdadm --stop /dev/md0

The two steps above will automatically be performed when the system is shut down.

To restart the array, use the --assemble option:

# mdadm --assemble /dev/md0 /dev/sdb1 /dev/sdc1

mdadm: /dev/md0 has been started with 2 drives.

To configure the automatic assembly of this array at boot time, obtain the array's UUID (unique ID number) from the output of mdadm -D :

# mdadm -D /dev/md0

/dev/md0:

Version : 00.90.03

Creation Time : Thu Mar 30 02:09:14 2006

Raid Level : raid1

Array Size : 63872 (62.39 MiB 65.40 MB)

Device Size : 63872 (62.39 MiB 65.40 MB)

Raid Devices : 2

Total Devices : 2

Preferred Minor : 0

Persistence : Superblock is persistent

Update Time : Thu Mar 30 02:19:00 2006

State : clean

Active Devices : 2

Working Devices : 2

Failed Devices : 0

Spare Devices : 0

UUID : 5fccf106:d00cda80:daea5427:1edb9616

Events : 0.18

Number Major Minor RaidDevice State

0 8 17 0 active sync /dev/sdb1

1 8 33 1 active sync /dev/sdc1

Then create the file /dev/ mdstat if it doesn't exist, or add an ARRAY line to it if it does:

DEVICE partitions

MAILADDR root

ARRAY /dev/md0 uuid=c27420a7:c7b40cc9:3aa51849:99661a2e

In this file, the DEVICE line identifies the devices to be scanned (all partitions of all storage devices in this case), and the ARRAY lines identify each RAID array that is expected to be present. This ensures that the RAID arrays identified by scanning the partitions will always be assigned the same md device numbers, which is useful if more than one RAID array exists in the system. In the mdadm.conf files created during installation by Anaconda, the ARRAY lines contain optional level= and num-devices= enTRies (see the next section).

If the device is a PV, you can now reactivate the VG:

# vgchange test -a y

1 logical volume(s) in volume group "test" now active

The mdmonitor service uses the monitor mode of mdadm to monitor and report on RAID drive status.

The method used to report drive failures is configured in the file /etc/ mdadm.conf . To send email to a specific email address, add or edit the MAILADDR line:

# mdadm.conf written out by anaconda

DEVICE partitions

MAILADDR raid-alert

ARRAY /dev/md0 level=raid1 num-devices=2 uuid=dd2aabd5:fb2ab384:cba9912c:df0b0f4b

ARRAY /dev/md1 level=raid1 num-devices=2 uuid=2b0846b0:d1a540d7:d722dd48:c5d203e4

ARRAY /dev/md2 level=raid1 num-devices=2 uuid=31c6dbdc:414eee2d:50c4c773:2edc66f6

When mdadm.conf is configured by Anaconda, the email address is set to root . It is a good idea to set this to an email alias, such as raid-alert , and configure the alias in the /etc/ aliases file to send mail to whatever destinations are appropriate: