Re: Upgrading a software RAID

On Thu May 28, 2009 at 10:32:53AM -0400, Maxime Boissonneault wrote:

Robin Hill a écrit :

quoted

On Mon May 25, 2009 at 01:05:15PM -0400, Maxime Boissonneault wrote:

  

quoted

Hello,
I am using Ubuntu Hardy with 3x500GB drives and the following RAID
configuration :

/boot is on a 100MB RAID1
/ is on a 30GB RAID0
/home is on a 906GB RAID5

    

Ouch - why RAID0 for /?  If you lose a single drive then all the
configuration, etc. is down the drain.  I'd suggest rethinking this
while you're going through the rebuild process anyway.

  

That is why I do backups of / on /home regularily. I first did a RAID0 for 
performance. I did some testing on performance of RAID0,1,5 and RAID1 was 
pretty lousy (I put the results of my tests here : 
http://cqed.physique.usherbrooke.ca/~mboisson/htpc.php?sec=raid_test )
I guess I should have used a RAID5, but RAID1 seems like a terrible idea 
considering the really bad performances.

There's definitely something up with the RAID1 there - there's no way it
should be that much slower than a single disk (it ought to be about the
same speed).  There have been some recent fixes to RAID1, so a more
recent kernel may help as well.  RAID10 could also be worth considering
as an alternative.

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I want to replace the 3 drives by 3 1TB drives.

Here is how I planned to do it :
0- Backup my /home on some external disk.
1- backup / with something like :
sudo tar cvpzf /backup.tgz --exclude=/media --exclude=/proc
--exclude=/lost+found --exclude=/backup.tgz --exclude=/mnt --exclude=/sys
--exclude=/home /
mv /backup.tgz $1
    

You'll need to do the tar in single user mode (init 1) to ensure there's
no open files when you're backing up (or use a bootable CD).
  

I used this backup before to restore the system and it seemed to work 
pretty well.

Okay - this'll depend on what you're running - databases in particular
can have major issues with backing up while open.

quoted

An easier option (assuming you want to keep the arrays as currently
setup) would be to boot from a CD and do a full copy of each of the
500GB disks to the 1TB disks.  You'll then just need to resize the
last partition (assuming /home is set up on the last partition on the
disks anyway) and grow the array and filesystem.

A _better_ option (if possible) would be to install all the drives in
the system concurrently, then you can boot from CD and create arrays on
the new drives and copy the data across.  You'll also need to update
mdadm.conf (and the initrd if you're using one) to indicate the new
array IDs.

I can not install more drives in the computer. It is a home theater 
computer in a small case. I was expecting to be able to let the raid manage 
the copies itself.

If you've got the ports (and power), you could just leave the case open
and the disks outside until you've finished the copy.

If the / was on a RAID5, would it be able to boot with 2 disks ?
If so, is it possible to convert my RAID0 to a RAID5 ?
For example, I could boot on a CD, backup / onto /home, delete the RAID0 
array and recreate it as RAID5, then restore the backup. Would this work ?

Yes, a RAID-5 will run fine with just two disks. You could certainly
boot from CD, backup, recreate the RAID, then restore - you'd probably
need to update the mdadm.conf file (and the initrd) to indicate the new
array UUID and details though.

You may also want to read through the GRUB section on
http://en.gentoo-wiki.com/wiki/RAID/Software to see how to install grub
onto all the drives (avoiding the issue of which drive is currently
being booted from).

Cheers,
    Robin
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