Linux LVM (mirroring)

[aldi]

Has anyone used software mirroring in Linux before? What's it like when things go wrong, just plug either of the disks into another machine and read it straightforwardly? I've been in situations with Windows software mirroring where you just get a barrage of cryptic error messages when you try to mount one of mirrors, what I suppose I'm getting at is does that happen with any regularity with Linux?

Cheers
Neil

[zumbruk]

www.tldp.org/HOWTO/Software-RAID-HOWTO.html

(Sorry, I can't be more helpful - I only know about Solaris Volume Manager for software RAID, and that's the dog's dangly bits.)

[Pigeon]

Mirroring is RAID, not LVM. The idea is that if one of the disks goes down you don't plug the other one into another machine, you can carry on using it in the same machine while you order a replacement for the knackered one, then you replace the knackered one when the new one arrives and it automatically rebuilds the mirroring so you're now safe if the remaining old one goes down.

Reports are that it works better with SCSI than IDE, and if you must use IDE put the disks on different buses, not as master and slave.

LVM is a different thing, it's for making a bunch of small disk drives look like one big one.

You can combine the two if you want to get complicated

[aldi]

So LVM != RAID... OK thats cool. I was more interested in what happens in say a motherboard faliure scenario where you want to dump the data onto a different box, I've had some real ball-aches doing that with windows s/w mirroring where windows won't mount the volume... just once or twice but enough to make me wary!

[zumbruk]

Pigeon said:

Mirroring is RAID, not LVM. The idea is that if one of the disks goes down you don't plug the other one into another machine, you can carry on using it in the same machine while you order a replacement for the knackered one, then you replace the knackered one when the new one arrives and it automatically rebuilds the mirroring so you're now safe if the remaining old one goes down. Reports are that it works better with SCSI than IDE, and if you must use IDE put the disks on different buses, not as master and slave. LVM is a different thing, it's for making a bunch of small disk drives look like one big one. You can combine the two if you want to get complicated

On AIX and Solaris, software RAID is done through the LVM. There's no other way of doing it, so your statement that RAID != LVM is not entirely true.

And it's always wise to put the mirrors on different buses/controllers, for performace reasons, no matter what the underlying technology is.

[Pigeon]

zumbruk said:

On AIX and Solaris, software RAID is done through the LVM. There's no other way of doing it, so your statement that RAID != LVM is not entirely true.

Yeah, but we're talking about Linux here

zumbruk said:

And it's always wise to put the mirrors on different buses/controllers, for performace reasons, no matter what the underlying technology is.

Agreed, but it's a lot more important with IDE than it is with SCSI - I think it's generally reckoned that four drives per bus is OK with SCSI before the performance starts to drop significantly, and certainly the SCSI drive rack I have here groups the drives into four on each bus.

My point wasn't so much performance as reliability however... if you have two IDE drives on the same bus and one of them pops it can screw up the other one. Doesn't tend to happen with SCSI.

aldi said:

So LVM != RAID... OK thats cool. I was more interested in what happens in say a motherboard faliure scenario where you want to dump the data onto a different box, I've had some real ball-aches doing that with windows s/w mirroring where windows won't mount the volume... just once or twice but enough to make me wary!

I've never tried swapping drives between boxes running the NT-based line of Windoze, but I've tried it on the DOS-based ones and it can be an arseache even without RAID, because Windoze seems to base its LBA translation on what the BIOS does and different BIOSes do it slightly differently so when you're using a different BIOS Windoze can't find its way around the drive any more. Linux ignores the BIOS once it's booted, so you can swap drives between Linux boxes with gay abandon. I haven't tried swapping a whole RAID set between Linux boxes but I wouldn't expect it to be a problem.

[tvrforever]

Veritas Foundation Suite for Storage = muts nuts.... and with PDCs you can mount the disk in various other OSes and still read the VM, FS & data

Swear by it