topic Re: Software Raid in Linux in Operating System - Linux

Software Raid in Linux

Admin32 — Mon, 26 May 2003 06:40:19 GMT

Greetings everyone,

I would like to implement a raid system on one of my Linux servers and would like to clarify a few things.

I read a few manuals online about software raid and they have left me with the impression that in order to create a raid 1 (mirror or duplex) you are required to erase the data on the partition you wish to include in the raid, setup the raid system and then restore the data.

Is this true ?

I am using Redhat v8 and if you happen to have some tips or any great website that I can read through, it would be appreciated.

Thanking you all in advanced.

Re: Software Raid in Linux

Christoph Rothe_3 — Mon, 26 May 2003 07:17:07 GMT

Hi,

yes thats true. You have to change the partition type and if you initialize the raid a header will be written onto the partition which erases a part of the data.

You will have to format (mkreiserfs, mkext2 or whatever) and restore your data.

Christoph

Re: Software Raid in Linux

Christoph Rothe_3 — Mon, 26 May 2003 07:18:01 GMT

http://linas.org/linux/Software-RAID/Software-RAID.html

Re: Software Raid in Linux

Kjartan Maraas — Mon, 26 May 2003 07:24:51 GMT

I think you are right that creating a raid set is a destructive action. The filesystem is created after the raid set is initialised so you can't "transfer" an existing filesystem onto the RAID-set. I guess the same is true for a first time setup with LVM, but there are advantages with LVM that makes it easier the next time you need to shuffle partitions around. Read more at http://tldp.org/HOWTO/LVM-HOWTO/index.html

HTH.

Re: Software Raid in Linux

Jarle Bjorgeengen — Mon, 26 May 2003 12:49:26 GMT

although Linux LVM doesn't contain mirroring yet. (As it does in hp-ux if you pay for it)

Use md devices (linux software raid) to build up the LVM volumegroups in order to get both.

Yes, it is a destructive action to make a raid . All data (or at least metadata) will be lost on partitions you use with raid devices. Make a backup before restore on the filesystem you craete on top of the md devices.

Rgds Jarle

Re: Software Raid in Linux

Jarle Bjorgeengen — Mon, 26 May 2003 12:51:47 GMT

Sorry , small typo.

=======
Make a backup before restore on the filesystem you craete on top of the md devices.
=======

should be
=======
Make a backup before creation of raid device. Restore on the filesystem you create on top of the md devices.
=======

Re: Software Raid in Linux

Admin32 — Mon, 26 May 2003 14:09:02 GMT

So from what I understand, there is no way of creating a RAID 1 for the system partition of a server after you have installed it - which is a bummer, cause on a live production server, this means you can't do much without reinstalling the whole darn thing !

Re: Software Raid in Linux

Jerome Henry — Mon, 26 May 2003 14:34:54 GMT

Yes,
you got it. Linux is still not HP-UX as far as efficiency is concerned. System is considered to be installed with or without raid, 'cause if you have 2 HD, then you put raid, and if you re-install andother hd thenafter, then you'll have to turn the thing off anyway (so it's not your production server anymore !)...
I'm dreaming of a partition magic like tool that would allow to resize partitions like fsadm, but it doesn't exist yet...
What you have to o now is turn it off, of forget about raid... :-((
J

Re: Software Raid in Linux

Jarle Bjorgeengen — Tue, 27 May 2003 06:56:48 GMT

Well,

all you need is to buy some new disks to put your raid on, make the raid, and copy across your operating system files.

You probably have to fiddle around a bit with modules.conf/initrd(md driver...) , disk device names, grub/lilo, but you then you can do all this while still being able to fall back on your old working boot disk.

I think your favorite Linux consultant would be happy to do it for you.

I have done it with ordinary harddisks, wgen transitioning from ext2 to reiserfs on my OS-filesystems. Just need to get the MBR , bootloader and fstab right. (Tip: use chroot to work on your mounted environment after copying across all files)

Rgds Jarle

Re: Software Raid in Linux

Gianluca Cecchi — Tue, 27 May 2003 11:00:13 GMT

You can also check at

http://www.tldp.org/HOWTO/mini/Boot+Root+Raid+LILO.html

especially at chapter 4 for a possible implementation, also mantaining old disk and trying the new configuration, before wiping it out.
HIH

Gianluca Cecchi

Re: Software Raid in Linux

Ralf Puchner — Tue, 27 May 2003 12:01:43 GMT

There is no automation tool to convert a non-raid to a raid partition. But you can install Linux on a raid partition (see Suse or Redhat).
Using softraid on boot and data partitions works like a charme if using the install option.

Btw. if installing a production machine requires a "plan" and not to install what you see along the road. ;-)