Linux RAID-EN
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Introduction
mdadm is one of the most advanced Linux software for the software array management.
Configuration
One of the main configuration files of mdadm is "/etc/mdadm.conf" that allows to trace software RAIDs.
For example, if the array doesn't start, the mdadm.conf file is useful because allows to determinate the array properties. However, generally is better to create the mdadm.conf file because, without it, every time that we execute the 'mdadm' command for raid management/activation, we should specify more information on the array.
It's possible to create mdadm.conf file using a text editor (for example nano, vi, mcedit, etc.) and adding the following directives, adapting them to our needs:
DEVICE /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 ARRAY /dev/md0 devices=/dev/sdc1,/dev/sdd1 ARRAY /dev/md1 devices=/dev/sde1,/dev/sdf1
On the first row "DEVICE" are specified the disks that will be part of the array. The second row "ARRAY /dev/md0..." create a first array composed of sdc1 and sdd1 disks. The third and last row "ARRAY /dev/md1..." create a second array composed of sde1 and sdf1 disks.
Alternatively we can use the 'mdadm' command for the file mdadm.conf creation:
# mdadm --detail --scan > /etc/mdadm.conf
Array Creation
The partitions that are part of an array must have the filesystem ID partition type setted on "FD". This can be done using a partitioning tool, like 'fdisk', 'qpart', 'cfdisk' or any other.
Create a "RAID-0" on "/dev/md0", using the partitions "/dev/sdb1" and "/dev/sdc1":
# mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/sdb1 /dev/sdc1
or using the short version of the same command:
# mdadm -Cv /dev/md0 -l0 -n2 -c128 /dev/sdb1 /dev/sdc1
Start an Array
If the file "mdadm.conf" has been created, the command will be the following:
# mdadm -As /dev/md0
Note: (-A assembles, s scan)
If otherwise you have NOT created the "mdadm.conf" file, the command will be the following:
# mdadm -A /dev/md0 /dev/sdb1 /dev/sdc1
Stop an Array
# mdadm -S /dev/md0
Add a disk to the array
If we want to add a disk sdh1 to the array md0:
# mdadm /dev/md0 --add /dev/sdh1
Remove a disk from the array
If we want to remove the sdh1 disk from the md0 array:
# mdadm /dev/md0 --fail /dev/sdc1 --remove /dev/sdc1
Monitor an Array
Examine an array:
# mdadm -E /dev/sdb1
Monitor an array:
# mdadm --monitor --delay=300 /dev/md0
Get information on the executing array:
# cat /proc/mdstat
Notes
For further information on the mdadm sintax and options:
# man mdadm
External links
- What's RAID?: Link to the official wikipedia page on RAID
- mdadm website: Link to the mdadm official page
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