====== Mdadm ======

=====Restoring failed drive=====
In case the mdadm raid is somehow lost or if you move your hard-drives to another computer, you might have to "re-create" the raid array.

First, mdadm stores it's configuration on each drive, so let's try to have mdadm automatically find the array back by itself : 
<code bash>mdadm --examine --scan</code>

It should show the HDDs participating in the array, if it's the case, try to have mdadm automatically start the array : 
<code bash>mdadm --assemble --examine -v</code>

If for some reason some HDDs appear out of sync in the output of the previous command, there is one last chance.
For this, you must **be sure that the array was clean** the last time it was disconnected !
First stop the (failed) array created earlier : 
<code bash>mdadm --stop /dev/md0</code>
Then **force**-create the array :
<code bash>mdadm --create --force /dev/md0 /dev/sd[acd]1</code>
This should do the trick, if not, search the internet for more ideas...

=====Improving rebuild-speed=====
===Tip #1: /proc/sys/dev/raid/{speed_limit_max,speed_limit_min} kernel variables==
This tip is **not** very efficient on my setup.
<code bash>
### Display the current values : ###
sysctl dev.raid.speed_limit_min
sysctl dev.raid.speed_limit_max

### Modify them : ###
sysctl -w dev.raid.speed_limit_min=1000
sysctl -w dev.raid.speed_limit_max=200000
</code>

===Tip #2: Set read-ahead option===
This tip **is very efficient** on my setup.
<code bash>
### Set read-ahead to 32 MiB ###
blockdev --setra 65536 /dev/md0
</code>

===Tip #3: Set stripe-cache_size for RAID5 or RAID 6===
<code bash>
echo 32768 > /sys/block/md0/md/stripe_cache_size
</code>

===Tip #4: Disable NCQ on all disks===
This tip is not useful since NCQ (Native Command Queing) is not supported by my hardware...
<code bash>
for i in sd[[abcde]]
do
  echo 1 > /sys/block/$i/device/queue_depth
done
</code>

===Tip #5: Bitmap Option===
This tip does **not work** when the array is already in resync state !
<code bash>
### To enable it : ###
mdadm --grow --bitmap=internal /dev/md0
 
### Once complete, to disable it : ###
mdadm --grow --bitmap=none /dev/md0
</code>

(Source : http://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html)