RAID5 + 1 extra disk HOW

[roland]

On Sat, 11 Oct 2008 23:56:34 +0200, Bill Davidsen <davidsen at tmr.com> wrote:

> 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.

I have to mention that the raid5 was build with the IBM utilities on this  
server x3500. So I suppose this mdadm wouldn't work. Or am I wrong?
Secondly, I need extra space only as a workarea.
That's why I thought, maybe I can:
- insert a 4th disk and add a filesystem, that isn't part of the raid5.
or
- insert a 4th and 5th disk and configure it as a raid1, to be used for  
non critical applications

In case of failure of the raid5, I could could use the 4th disk to replace  
the failing disk of the raid5 and let the ibm utilities rebuild the raid5

Does this makes sense?

>
>> 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.
>



-- 
Roland Brouwers
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