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To manage the RAID SOFT, we will use mdadm.

Attention: IT IS DANGEROUS TO PLAY WITH MDAM IF YOU DON'T KNOW WHAT YOU'RE DOING. YOU RISK LOSING THE DATA. BACKUP BEFORE READING OF THIS MANUAL

Note:
If your server just had its disk changed, then you have a disk in RAID soft and a blank disk that contains no partition table.

You must then copy the partition of the disk to the new one.

In case of SDA change:

sfdisk -d /dev/sdb | sfdisk /dev/sda

In case of SDB change:

sfdisk -d /dev/sda | sfdisk /dev/sdb

Note:
sfdisk does not support GPT partition tables, which will becoming more popular (for example it is default in XenServer? 6)

So if the server has GPT and Soft RAID, you use:
In case of SDA change:

sgdisk -R=/dev/sda /dev/sdb

In case of SDB change:

sgdisk -R=/dev/sdb /dev/sda


How to see list partitions on a disk?
cat /proc/partitions

or
parted /dev/sdX print


1.) MDADM installation

The rest of the MDADM softwares is not installed by default on the server.

In release 1:

rpm -Uvh --nodeps ftp://ftp.ovh.net/made-in-ovh/sources/mdadm-2.5.5p1-1.i386.rpm
Retrieving ftp://ftp.ovh.net/made-in-ovh/sources/mdadm-2.5.5p1-1.i386.rpm
Preparing... ########################################### 100%
1:mdadm ########################################### 100%



2.) Help

To further information use --help

# mdadm --misc --help
Usage: mdadm misc_option devices...

This usage is for performing some task on one or more devices, which
may be arrays or components, depending on the task.
The --misc option is not needed (though it is allowed) and is assumed
if the first argument in a misc option.

Options that are valid with the miscellaneous mode are:
--query -Q : Display general information about how a
device relates to the md driver
--detail -D : Display details of an array
--examine -E : Examine superblock on an array component
--examine-bitmap -X: Display contents of a bitmap file
--zero-superblock : erase the MD superblock from a device.
--run -R : start a partially built array
--stop -S : deactivate array, releasing all resources
--readonly -o : mark array as readonly
--readwrite -w : mark array as readwrite
--test -t : exit status 0 if ok, 1 if degrade, 2 if dead, 4 if missing



3.) Information

To get information about the status of the RAID use --misc command.


  1. mdadm --misc --detail /dev/md1
/dev/md1:
Version : 00.90.00
Creation Time : Tue Nov 7 22:01:16 2006
Raid Level : raid1
Array Size : 3068288 (2.93 GiB 3.14 GB)
Device Size : 3068288 (2.93 GiB 3.14 GB)
Raid Devices : 2
Total Devices : 1
Preferred Minor : 1
Persistence : Superblock is persistent

Update Time : Wed Nov 8 15:42:35 2006
State : active, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0

UUID : 4a9a446d:af55e24b:b311aa61:8dc74ed4
Events : 0.12

Number Major Minor RaidDevice? State
0 8 1 0 active sync /dev/sda1
1 0 0 1 removed


We see that we have a RAID 1 of 3 Gb, which is in degraded mode because the sdb1 disk is missing.

4.) ADD is a disk in degraded mode.

Here is an example of commands, to add a disk to a raid in degraded mode:


  1. mdadm /dev/md1 --manage --add /dev/sdb1
mdadm: hot added /dev/sdb1


The raid in rebuilding.


  1. cat /proc/mdstat
Personalities : linear raid0 raid1 raid5
read_ahead 1024 sectors
md1 : active raid1 sdb12 sda10
3068288 blocks 2/1 U_
=>................... recovery = 5.4% (167028/3068288) finish=0.8min speed=55676K/sec
md2 : active raid1 sda20
240597376 blocks 2/1 U_



A few minutes later the reconstruction will be finished:


  1. mdadm --misc --detail /dev/md1
/dev/md1:
Version : 00.90.00
Creation Time : Tue Nov 7 22:01:16 2006
Raid Level : raid1
Array Size : 3068288 (2.93 GiB 3.14 GB)
Device Size : 3068288 (2.93 GiB 3.14 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 1
Persistence : Superblock is persistent

Update Time : Wed Nov 8 16:01:02 2006
State : active
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

UUID : 4a9a446d:af55e24b:b311aa61:8dc74ed4
Events : 0.14

Number Major Minor RaidDevice? State
0 8 1 0 active sync /dev/sda1
1 8 17 1 active sync /dev/sdb1



We will do the same on md1:


  1. mdadm /dev/md2 --manage --add /dev/sdb2
mdadm: hot added /dev/sdb2


Rebuilding:


  1. mdadm --misc --detail /dev/md2
/dev/md2:
Version : 00.90.00
Creation Time : Tue Nov 7 22:01:16 2006
Raid Level : raid1
Array Size : 240597376 (229.45 GiB 246.37 GB)
Device Size : 240597376 (229.45 GiB 246.37 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 2
Persistence : Superblock is persistent

Update Time : Wed Nov 8 16:06:43 2006
State : active, degraded, recovering
Active Devices : 1
Working Devices : 2
Failed Devices : 0
Spare Devices : 1

Rebuild Status : 0% complete

UUID : e8754328:26f28599:8b834aba:ade37773
Events : 0.10

Number Major Minor RaidDevice? State
0 8 2 0 active sync /dev/sda2
1 0 0 1 removed

2 8 18 2 spare /dev/sdb2

  1. cat /proc/mdstat
Personalities : linear raid0 raid1 raid5
read_ahead 1024 sectors
md1 : active raid1 sdb11 sda10
3068288 blocks 2/2 UU

md2 : active raid1 sdb22 sda20
240597376 blocks 2/1 U_
>.................... recovery = 0.2% (655104/240597376) finish=73.2min speed=54592K/sec
unused devices: <none>




Check the size of your swap at the end.


cat /proc/swaps
Filename Type Size Used Priority
/dev/sdb3 partition 522104 43984 -1


You must now add the swap of the disk that you have just formatted and re-added to the RAID.

mkswap /dev/sda3
swapon -a

You can see with the free command that the whole swap space is activated again.