partitioning new installation

Thu Mar 9 22:08:53 UTC 2006

Quoting Lennart Sorensen <lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org>:

> On Thu, Mar 09, 2006 at 04:08:47PM -0500, wattst-dxuVLtCph9gsA/PxXw9srA at public.gmane.org wrote:
> > I'm curious about LVM as I should be doing a reinstall soon.  I understand
> that
> > that LVM maps physical entities to appear as one or many different volumes
> > which don't neccessarily reflect the geometry of the physical.  What I
> don't
> > understand is when you say resizing the volumes is easy.  Does it make in
> > simpler because you can grow an LVM over existing, formated space?  Or
> shrink
> > it and create a new volume on the newly freed space?
> >
> > What I'm really getting at is what has to happen underneath the LVMs.  Do
> you
> > already have to have a fixed chunk formated as an LVM compatible filesystem
> and
> > then build on top of only that?  If I have an NTFS partition and then ext3
> > partitions with LVM on top and I want to give some space from NTFS to ext3,
> is
> > that possible?  What steps would be involved in doing that?
>
> The main problem with resizing when usingpartitions is this:
>
> Partition   Start sector   End Sector
> 1           0              999
> 2           1000           2999
> 3           3000           9999
>
> If you wanted to shrink partition 3 and increase partition 2 by 500,
> then you have to first shrink partition 3 by 500, then move it to the
> end of the device to leave 500 free between partition 2 and 3 and then
> expand partition 2 by 500.
>
> With LVM you don't worry about the physical blocks actually used by the
> logical volumes (LV) since that is all managed automatically.  You
> create some physical volumes (PV) and then create a volume group (VG)
> from those PVs.  The PVs can be whole disks, md raid devices,
> partitions, or any other kind of block device in linux.  You can
> combine any combination of PVs into a VG.  Once you have your VG, you
> create LVs inside it of whatever size you want.  Maybe you are not sure
> how big you want things, you can leave some unallocated for later
> expansions.  If you later want to change a logical volume, since you
> don't know or care where the LV physically stores it's blocks, you can
> reduce the size of an LV, and gain free space in the VG, and then expand
> another LV with some of that free space.  For all you know one LV uses
> the odd blocks and another the even blocks, although that is pretty
> unlikely.  It doesn't amtter. since LVM maintains the map of which
> physical blocks of all the PVs is used to store the blocks of each LV.
> It is quite complex internally, but very simple to use.
>
> The ability to make VGs from many devices is also handy for making
> things bigger.  For example I am about to add space to a server here, so
> I am going to get a pair of 250G drives, setup one partition on each and
> create a software raid1 on those.  Then I will run pvcreate on the new
> md device, and then use vgextend to add it to the existing VG I have.
> Then I will use lvextend to expand the existing LV for the /data
> partition I have file shares on, and once that is done, I will use
> e2fsresize most likely to resize the ext3 filesystem to fill the new
> size of the LV.  So even though I now have two raid1s on different
> disks, the LV will span over both of them and appear to the system as a
> single large filesystem.  It doesn't matter to the users that it does
> this, they just see the filesystem being much bigger now and have more
> free space.  I might leave 50G or so unallocated in the VG after
> expanding, just to leave room for increasing other things, although most
> likely not, since this is the only volume that has ever gotten close to
> full.
>
> Len Sorensen
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Thanks for the reply.

Okay, so let's say I typically have two LVs on one VG; one for / and one from
/home as that's the way I would usually partition.  Now, when I go to reinstall
the OS, will I be able to simply install to / leaving /home unaffected?

Finally, when creating a new VG from existing partitions, is it possible to keep
the data on those partitions and use that data in a new LV (ending up with
essentially the same structure, only now with LVM)?

Tom Watts
wattst-dxuVLtCph9gsA/PxXw9srA at public.gmane.org

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