RAID5 + 1 extra disk HOW

[Bill Davidsen]

roland wrote:
> Hello,
> 
> I have a  server running on RAID5, on which I would like to add a 4th 
> disk, with a seperated filesystem. I need to have some workspace. The 
> same disk could be used to replace a failing disk.
> 
This is a spare disk, and can be used only as such. I'm not sure what you had in 
mind by "seperated filesystem," but you can't install another drive such that 
Linux will use the drive in two ways at once. If that was your intent it pretty 
much isn't going to work, and if the spare was deployed all data on it would be 
lost.

> Stuart Sears explaned me howto install this disk as a hot spare, but it 
> looks to me a little bit to dangereous. This server is serving 20 
> workstations, so I need to do this without taking to many risks. I admit 
> to be a novice on the matter of diskmanagement. I always do standard 
> installation of a filesystem.
> 
Assuming you just want to add a spare, something like this works fine:
	mdadm --add /dev/md0 /dev/sdf1
Then a cat of /proc/mdstat should show the device added, running as a spare. I 
did that, then failed one of the existing drives, and the array rebuilt on the 
spare just fine, and I got a message from monitor mode saying it happened and 
was fixed.

> So if someone could tell me howto do this in a simple way. I searched 
> the internet for a simple howto, but was unsuccessful. I have to do this 
> at a client in France, few hours driving
> 
> Could someone keep me from having a sleepless night?

I would test the existing array before counting on it to be okay, just in case a 
bad spot developed.See the wiki (http://linux-raid.osdl.org). Oh, and there is a 
linux-raid mailing list where there are lots of us who have found problems the 
hard way and want to share.

-- 
Bill Davidsen <davidsen at tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot