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Re: Software Raid

Subject: Re: Software Raid
From: Gavin McCullagh
Date: Fri, 1 Aug 2008 10:47:17 +0100
Hi,

On Thu, 31 Jul 2008, Uwe Geercken wrote:

> at the time I setup the system, I have used one harddisk for the  
> system and one for the home directories of the students. so these are  
> on seperate drives.

What sizes are the respective drives?  Are they IDE, SATA, SCSI?

> I am now thinking putting in a second drive and use software raid so  
> that I will have faster reads. my first thought was to use raid for  
> the system and still have the home folders on a seperate drive. or  
> maybe I should put home and the system on the same drive and use raid  
> for both?

I presume you're talking about MD (linux kernel software raid).  Presumably
it can read off the disks independently so you should probably get improved
read speeds.  To my mind though, reliability is the best feature of RAID1.

http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_1_performance

If you can get both the system and the home directories onto RAID1, you're
system will keep running in the event of a single disk failure.  Disk
failure is remarkably common these days.

Migrating the running system to software RAID is not easy, so you may find
it easiest to reinstall which is a bit of a downside.  Presumably you can
just back up and restore the home directories with tar.  For the system, if
you have sufficient space and don't wish to reinstall, you could possibly
do something like

1. Shrink your / partition by 50% using gnu parted
2. Create a new equal sized partition in the resulting space.  Make it of
   type linux-raid
3. Create a larger raid partition on the new disk.
4. Use mdadm to create a raid1 array using the two raid partitions.
5. Format the raid partition ext3 (or whatever you're using)
6. Reboot and make sure the raid partition comes back up.
7. Drop to single user mode and use something like cpio or rsync to sync
   the new raid partition with the old one.
8. Configure grub with extra boot items to boot onto the new partition
9. Reboot using the new grub entry and say a prayer (if it doesn't work,
   you should be able to boot onto the old partition anyway).
10.Once you're up on the new partition, you can re-run grub-install,
   delete the old non-raid partition, expand the existing one back into the
   freed space, expand the raid array into the space and finally expand the
   root filesystem on the bigger partition.

These instructions are untested and I've probably missed something.
Frankly, I'd highly recommend the reinstall, backing up your config first
of course!

Gavin


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