====== Linux Software RAID ======

http://wiki.centos.org/HowTos/SoftwareRAIDonCentOS5

http://wiki.xtronics.com/index.php/Raid

http://robbat2.livejournal.com/231207.html

https://raid.wiki.kernel.org/index.php/Linux_Raid

http://archive.networknewz.com/2003/0113.html

Works on block devices, usually partitions.  This is not like strictly-drive-oriented hardware RAID controllers.

===== Show Existing RAID =====

<file>
mdadm --detail --scan --verbose
</file>

===== Partitioning =====

<file>
fdisk -l /dev/sda
</file>

or

<file>
sgdisk -p /dev/sda
</file>

**Copy Patition Scheme**: https://askubuntu.com/questions/57908/how-can-i-quickly-copy-a-gpt-partition-scheme-from-one-hard-drive-to-another/333923

:!: Verify ''sfdisk'' recognizes your source partitions.  If not, try ''sgdisk''.

Using fdisk, create matching partitions on various drives (block devices) of type **fd  Linux raid autodetect** that will be assembled into arrays.

<file>
sfdisk -l /dev/sda

Disk /dev/sda: 8942 cylinders, 255 heads, 63 sectors/track
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

Device      Boot  Start     End    #cyls    #blocks  Id  System
/dev/sda1   *         0+    242     243-   1951866   fd  Linux raid autodetect
/dev/sda2           243     485     243    1951897+  fd  Linux raid autodetect
/dev/sda3           486     607     122     979965   82  Linux swap
</file>

:!: Be careful not to write to the **wrong disk** or you will have a **very bad day**!

Once you partition the first drive, use ''sfdisk'' or ''sgdisk'' to replicate partitions from that first drive to other drives:

<file>
sfdisk -d /dev/sda | sfdisk /dev/sdb
</file>

or

<file>
sgdisk /dev/sda -R /dev/sdb
sgdisk -G /dev/sdb
</file>

===== Create Arrays =====

http://robbat2.livejournal.com/231207.html

http://en.wikipedia.org/wiki/Mdadm

<file>
mdadm --create <[[array]]_device> -c <chunk_size> -l <raid_level> -n <active_disks> -x <spare_disks> <block_devices>
</file>

Here is a four-drive (partition) RAID5 example where one drive (partition) is a hot spare.  Hot spare drives can serve multiple arrays (spare-group).

<file>
mdadm --create /dev/md0 --level=5 --raid-devices=3 --spare-devices=1 --spare-group=groupname /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
</file>

===== Add a Drive =====

http://www.kernelhardware.org/linux-add-share-hot-spare-device-software-raid/

<file>
mdadm --detail --scan --verbose

mdadm --add /dev/md0 /dev/sdb1
mdadm --add /dev/md1 /dev/sdb2
mdadm --add /dev/md2 /dev/sdb3
</file>

===== Create Configuration File =====

Not sure how beneficial or necessary this step is...

<file>
mdadm --detail --scan > /etc/mdadm/mdadm.conf
</file>

or for Centos:

<file>
mdadm --detail --scan --verbose > /etc/mdadm.conf
</file>

===== MBR and GRUB =====

==== MBR ====

http://www.cyberciti.biz/faq/howto-copy-mbr/

Assuming you have identical disks with identical partitioning, use this command to copy the entire master boot record (boot code plus partition table) like this, adjusting for your needs:

<file>
dd if=/dev/sda of=/dev/sdb bs=512 count=1
</file>

==== GRUB ====

http://wiki.centos.org/HowTos/GrubInstallation

<file>
grub
</file>

If you have a separate /boot partition:

<file>
grub> find /grub/stage1
</file>

If you don't:

<file>
grub> find /boot/grub/stage1
</file>

Then, based on the output of the find command:

<file>
grub> root (hd0,0)
grub> setup (hd2)
</file>