====Managing software raid1 with mdadm====

===Description===
mdadm is a tool designed to replace raidtools under GNU/Linux.


===Requirements===
Linux with md devices support and RAID 1 support.

===Usefull commands===

^ Task ^ Command ^
| Create a new mirror | mdadm -C -n 2 -l 1 /dev/md0 /dev/sda /dev/sdb|
| Verify mirror status | mdadm --detail /dev/md0 |
| Hot remove a drive | mdadm --manage -r /dev/md0 /dev/sda |
| Hot add a drive | mdadm --manage -a /dev/md0 /dev/sda |
| Simulating a faulty disk | mdadm --manage /dev/md0 --set-faulty /dev/sda |

===Disk Failure===
If a disk fails, you should replace the disk and tell Linux to resync the array.
First, you'll need to __identify the faulty disk__. (using the dmesg for example)

Then, you should stop the Linux host, and replace the faulty disk.
Once the disks is changed, restart the host.

Now, you should reintegrate the new disk to the array:

(note: **you should replace sdc and md1 with correct values you've identified before**)

<code>
#> mdadm --manage /dev/md1 -r /dev/sdc
#> mdadm --manage /dev/md1 -a /dev/sdc
</code>

Now, enjoy reconstruction !

You could get informed on __sync status__:

<code>
#> mdadm --detail /dev/md1
</code>

Sample output:
<code>
/dev/md1:
        Version : 00.90.01
  Creation Time : Thu Mar 23 23:09:39 2006
     Raid Level : raid1
     Array Size : 160086464 (152.67 GiB 163.93 GB)
    Device Size : 160086464 (152.67 GiB 163.93 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 1
    Persistence : Superblock is persistent

    Update Time : Fri Mar 24 06:56:27 2006
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 6965fc49:6b5790d8:2ca8dc68:1afb7fbd
         Events : 0.14319

    Number   Major   Minor   RaidDevice State
       0       8       32        0      active sync   /dev/sdc
       1       8       48        1      active sync   /dev/sdd
</code>

===/etc/mdadm/mdadm.conf===
mdadm has an optional config file, which could be used to specify less informations
on the command line.

Sample config file:
<code>
DEVICE partitions
ARRAY /dev/md0 level=raid1 num-devices=2
   devices=/dev/sda1,/dev/sdb1
ARRAY /dev/md1 level=raid1 num-devices=2
   devices=/dev/sdc1,/dev/sdd1

MAILADDR your@mail.to.be.informed.in.case.of.disk.failure
</code>