Re: [PLUG] Lost gigabytes?

[JP Vossen <jp@jpsdomain.org>]

> Date: Thu, 8 Dec 2011 15:47:29 -0500
> From: Rich Freeman > <r-plug@thefreemanclan.net>

<snip>

> Your one partition is further divided up using LVM, and so root is
> just a portion of it.  LVM does allow for resizing so you can shuffle
> things around - IF YOU KNOW WHAT YOU ARE DOING!!!!!!!!!  You may need
> to shrink existing filesystems and then their corresponding logical
> volumes - then increase other logical volumes and their corresponding
> filesystems.

If you also mirror disks (I do), then this is really nice because you 
can migrate to bigger disks while the system is running, except for 2 
reboots for hardware swaps! I have done this a few times (e.g., 2010-11 
for my MythTV server), it's awesome.

To oversimplify quite a bit:
	0) Make sure your mirror is in sync
	1) Down and swap in bigger drive as #2
		# Save original drive #2, it's your backup!
	2) Re-mirror #1 to new bigger #2
	3) Down and swap in bigger drive as #1
	4) Resize (enlarge) the RAID and LVM devices
		# https://raid.wiki.kernel.org/index.php/Growing
	5) Re-mirror #2 to new bigger #1 (don't screw this up!)

Aside from the reboots, all of this happens while the system is live, 
though of course it'll run slower.

Some of the re-size commands:
	mdadm -v --grow /dev/md1 --bitmap none
		# Bitmaps BAD when resizing!
	mdadm -v --grow /dev/md1 --size max
	mdadm -v --wait /dev/md1
		# Waits until re-synced, takes hours
	lvresize --verbose -l +297350 /dev/vg_mythtv-be-01/lv_root
	resize2fs -p /dev/vg_mythtv-be-01/lv_root


> If this is on a laptop might not hurt to bring it to a PLUG meeting or
> otherwise work with somebody who is familiar with LVM.  Or you can
> learn about it online, but make sure you understand what is going on
> or (ideally and) have good backups.

The other nice thing about LVM is that it allows for "snapshots." 
Maybe.  The capability is there, but to actually use it requires that 
you have some "free extents" which you will not have if you did a guided 
setup in Ubuntu.  (I filed a bug about that a long time ago.) 
Basically, if you create a snapshot, the CoW (copy on write) data has to 
go somewhere, and if all space is already used, you're toast.

I set up LVM with about 2-3% of space free, so that I can use snapshots 
for backup purposes if I want to.  Having said that, I've never actually 
gotten around to *doing* that.  But the plumbing is all there, all I'd 
need to do script it.  :-)

As I understand it, all of the LVM type stuff will eventually go away 
if/when btrfs goes fully production.  But that's years out, as discussed 
elsewhere.

Later,
JP
----------------------------|:::======|-------------------------------
JP Vossen, CISSP            |:::======|      http://bashcookbook.com/
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