Unsure of changes to array

Unsure of changes to array

[Jonathan Gazeley]

Dear all,

This is hopefully a simple question for you to answer, but I am fairly 
new to RAID and don't want to risk losing my data!

My setup is as follows:
- I have four 500GB disks. Each disk is split into a 5GB partition, and 
a 495GB partition.
- The four 5GB partitions are in a RAID-5 array, md0. CentOS is 
installed on this 15GB partition.
- The four 495GB partiions are in a RAID-5 array, md1. This partition 
holds my user data.

However, I have decided I no longer wish to two arrays across the disks. 
I've added a fifth disk on which I have installed the OS without RAID, 
meaning the old md0 is currently unused. Can I simply remove the four 
5GB partitions, and resize the four 495GB partitions to fill the entire 
disk? Will this break anything?

Before anybody tells me off for having the OS on a non-RAID disk, this 
is a home server and therefore high availability is not an issue. But 
keeping my data safe against disk failures is important to me.

Cheers,
Jonathan

----------------------
Jonathan Gazeley
ResNet | Wireless Team
Information Services
----------------------

Re: Unsure of changes to array

[Nagilum]

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----- Message from jonathan.gazeley@bristol.ac.uk ---------
     Date: Wed, 17 Oct 2007 15:04:52 +0100
     From: Jonathan Gazeley <jonathan.gazeley@bristol.ac.uk>
Reply-To: Jonathan Gazeley <jonathan.gazeley@bristol.ac.uk>
  Subject: Unsure of changes to array
       To: linux-raid@vger.kernel.org


> Dear all,
>
> This is hopefully a simple question for you to answer, but I am fairly
> new to RAID and don't want to risk losing my data!
>
> My setup is as follows:
> - I have four 500GB disks. Each disk is split into a 5GB partition, and
> a 495GB partition.
> - The four 5GB partitions are in a RAID-5 array, md0. CentOS is
> installed on this 15GB partition.
> - The four 495GB partiions are in a RAID-5 array, md1. This partition
> holds my user data.
>
> However, I have decided I no longer wish to two arrays across the
> disks. I've added a fifth disk on which I have installed the OS without
> RAID, meaning the old md0 is currently unused. Can I simply remove the
> four 5GB partitions, and resize the four 495GB partitions to fill the
> entire disk? Will this break anything?
>
> Before anybody tells me off for having the OS on a non-RAID disk, this
> is a home server and therefore high availability is not an issue. But
> keeping my data safe against disk failures is important to me.
>
> Cheers,
> Jonathan
>

----- End message from jonathan.gazeley@bristol.ac.uk -----

***CAUTION***CAUTION***CAUTION***
Do backups if you can..
Are the RAID partitions on your disk the first partition or a different one?
If its not the first partition you'll have _move_ your partition to  
the beginning of the disk.
So assuming your first disk is /dev/sda, sda1 your OS partition which  
want to recycle, sda2 your RAID partition you could do:
  dd if=/dev/sda2 of=/lots_of_space_here/sda.dump
then repartition your sda, now sda1 is your RAID partition and it  
spans across the whole drive
  dd if=/my_new_disk/sda.dump of=/dev/sda1

After that you should be able to assemble your RAID again with  
/dev/sda1 instead of /dev/sda2. If that works, continue with the other  
disks.
If it doesn't work your array should be degraded and you should still  
be able to add /dev/sda1 and then md should sync to it to become clean  
again.
Note that during this whole operation you will be vulnerable to any  
disk failures. You may think about buying another disk and converting  
to raid6 for an additional layer of protection before attempting this.
If you have friend who has a faint idea what you do have him join you  
to look over your shoulder and stop you from doing silly mistakes  
(e.g. mixing up "if" and "of" during dd). This is a very dangerous  
undertaking make one mistake and your data may be gone...
I hope this helps..
Alex.

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Re: Unsure of changes to array

[Bill Davidsen]

Jonathan Gazeley wrote:
> Dear all,
>
> This is hopefully a simple question for you to answer, but I am fairly 
> new to RAID and don't want to risk losing my data!
>
> My setup is as follows:
> - I have four 500GB disks. Each disk is split into a 5GB partition, 
> and a 495GB partition.
> - The four 5GB partitions are in a RAID-5 array, md0. CentOS is 
> installed on this 15GB partition.
> - The four 495GB partiions are in a RAID-5 array, md1. This partition 
> holds my user data.
>
> However, I have decided I no longer wish to two arrays across the 
> disks. I've added a fifth disk on which I have installed the OS 
> without RAID, meaning the old md0 is currently unused. Can I simply 
> remove the four 5GB partitions, and resize the four 495GB partitions 
> to fill the entire disk? Will this break anything?

Not unless you make a mistake in typing, or your hardware or power fails 
during the process. Of course in that case you will possibly lose 
everything.
>
> Before anybody tells me off for having the OS on a non-RAID disk, this 
> is a home server and therefore high availability is not an issue. But 
> keeping my data safe against disk failures is important to me.

Having your o/s and swap on RAID reduces your chances of losing your 
data. If it were me and reliability were the goal, I would have the o/s 
mirrored on the first two drives (as seen by the BIOS) so you boot if 
one fails hard, then put swap RAID-10 in the little 5GB partition on the 
other three drives, then convert the raid-5 to raid-6 using the rest of 
the added fifth drive.

Anything which reduces the chances of an unclean shutdown improves the 
chances of keeping your data. A decent UPS is a big help in that regard. 
Disk failures on the data drives are not the only threat to your data!

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979