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Re: extra harddisk on RAID5

On Tue, 30 Sep 2008 12:32:55 +0200, Stuart Sears <stuart sjsears com> wrote:

roland wrote:

I have a server with 3 Harddisks 74Gb Raid5
I would like to install a 4th disk as a backup.
I never did this before. What will happen when I plug this disk in,

you wish to add the 4th device as a hot spare to your RAID5?

will it install itself?
how will it appear?
When one of the disks should fail in the future, can I pull out this 4th
disk and plug it in as replacement for the failing disk?
Or there some things I have to take care of?

1. partition it in exactly the same way as the other disks.
Assuming It's a single large partition, make sure that the partition is
labelled as 'fd' (or Linux RAID autodetect).

2. run 'partprobe' to inform the kernel about your new partition(s)

3. Add it to your RAID5 as a hot spare to the raid array (assuming it's
called /dev/md0):

mdadm /dev/md0 -a /dev/sdd1

(assuming the new disk is /dev/sdd and you have a partition on it called

4. Now
mdadm --detail /dev/md0
should show you have 4 devices, one is a spare device. This will
automatically be used to replace a failed disk.

If you want to remove a failing disk now (/dev/sdb1 for example) you can

5. fail and remove the broken device:
mdadm /dev/md0 -f /dev/sdb1
mdadm /dev/md0 -r /dev/sdb1

6 let the array rebuld itself. You can see this happening in /proc/mdstat:
watch 'cat /proc/mdstat'

7. You should now be able to physically remove the failed device (you'll
probably have to shut down for this unless you have hotpluggable disks)

Thanks for this detailed reply, but I have to tell you that I am quite novice with partitioning, because I always take the automatique partitioning.

Actually I install vmware server on this Raid5 with 3 virtual clients. Because there is not enough space anymore on this raid, I would like to install a new bigger virtual client on the new disk, copy everything from the old virtual client to the new one, remove the old and copy the new one to the raid and keep the disk as spare.

Of course if this 4th disk can be part of this raid5 I will have the extra space.
BUT, what will happen when one of the disks fail, I will have no spare.

Probably I am understanding you wrongly.

So in one word I need extra space and a spare disk.


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