Paritions to install Ubuntu

[Alan McKinnon]

On Monday 02 October 2006 10:21, Johann Spies wrote:
> > LVM is a marvellous tool, designed to solve a horrendous
> > problem - physical partition sizes end up being fixed at
> > install time (for all practical purposes) and it's very
> > hard to change them. It can be done, but it gets to be like
> > a Rubik cube or that 15-pieces kid's game. LVM gives you a
> > *virtual* partition, so instead of a /dev/sda6 you get a
> > /dev/vg/home. In use, the LVM acts exactly like a physical
> > partition except that you can change it's size dynamically
> > (to reduce the size you need to unmount first though. Not
> > so to enlarge ext2/3 or reiser3). I dual boot two OSes on
> > this notebook, both use LVM and it works seamlessly.
> > Install it, use it and mostly forget about it till you need
> > to change partition sizes.
>
> Be careful!  LVM has bitten me more than once.  I think one
> has to know the limitations of LVM very well before trying it
> out.  Planning is crucial.
>
> Example 1: I recently did two installations using LVM.  In
> one case I had a 250G disk so I used a small /boot partition
> (ext2) and the rest LVM. Later I wanted to free some space
> for a separate partition to experiment with Xen.  I could not
> do it.

Your mistake was in having one huge physical partition as the 
entire LVM system, and then trying to reduce the size of the 
resulting pv. LVM is much more flexile than that :-)

There's two easy ways around that:

1. Create new lvs in the vol group for the new distro. For all 
intents and purposes they act the same as physical partitions 
and co-exist with the existing ones nicely

2. When partitioning the physical disk, divide it up into 10 
partitions of about 25G each. Make then all pvs and add them to 
the same volume group. If you really do need a different 
partition later that's not on lvm, remove the relevant pv from 
the vg, freeing it up for non-lvm use

But as you say, it's good to know these things beforehand and to 
be able to predict future scenarios

alan