Re: [gentoo-user] LVM (Was: the best filesystem for server: XFS or JFS (or?))

[Alan McKinnon]

On Saturday 26 March 2011 15:36:19 Mark Knecht wrote:
> Dale,
>    I understand your position and concerns. While I have a number of
> systems, I have little time or patience for dealing with a lot of this
> stuff and LVM has been one of them.
>
>    One thing I'm considering to try out LVM is a second Gentoo
> installation on an already running system. It will either be a 50GB
> partition of its own, or a Virtualbox VM. I'd do the normal Gentoo
> install for LVM, figure out how it works, etc., and then decide if I
> want to use it in the future.

Well I can help with that, or at least provide some tips. Delivering Red Hat's 
training courses exposes you to all the weird and wonderful ways people 
misunderstand LVM and the even weirder ways gnome tools present the subject...

Logically, LVM sits between your physical disks (or raid arrays if you use 
that) and the filesystem. All it is is a way to manipulate these things called 
"block devices" in ways that you normally can't do without LVM.

Like if you have 4 partitions on a disk and want to make the third one bigger. 
Using just fdisk, you can't do that without making backups and restoring. Or 
creating a filesystem larger than any one disk.

So LVM takes a bunch of disks or arrays and lets you combine them in ways you 
want them (not ways the hardware forces you to have them). And that's all it 
does - forget all the nonsense in the man pages about aligning stripes to make 
mirrors - that just confuses people and makes them think it's some fancy raid.

You could argue that LVM exposes too much complexity  by letting you see the 
physical volumes (pv), volume groups (vg) and logical volumes (lv), and I 
won't argue with that. It's a trade between flexibility and complexity. I'm 
happy with it, others might not be.



-- 
alan dot mckinnon at gmail dot com