[CentOS] Some basic LVM questions

M. Hamzah Khan hamzah at hamzahkhan.com
Sun Nov 8 19:51:01 UTC 2009

On Sun, 2009-11-08 at 10:44 -0800, Monte Milanuk wrote:
> M. Hamzah Khan wrote:
> > There won't be any issue in doing this. The installer just tries to make
> > things easier by creating one big volume group.
> >
> > I'd say that in some ways seperating the two disks in this case would
> > actually be better. :)
> >   
> My last 'serious' experience with Linux was some years ago... mostly 
> before LVM really became popular (it was out and about, but mostly only 
> in SuSE).  I'm still 'stuck' in the mind set of a main drive or 
> partition for things like '/', possibly even /boot, /var, /usr, etc. and 
> then keeping /home separate - mainly so the user data in /home survives 
> upgrades and updates and such ;)

I actually ran into this recently too! 3TB volume group, with very old
backups, and no RAID. One drive failed, almost lost all of the data. I
had to send the drives off to a data recovery center and get the data
recovered professionally.

> > With both drives in one big volume group, failure of one drive will
> > (most likely) cause both the OS and data to be lost. 
> >   
> There in lies some of my confusion with this subject; correct me if I'm 
> wrong in my understanding here: with LVM, I can keep adding more drives 
> to a 'pool' and expand the size of the 'volume' that the OS sees 
> available to it... but if any drive in that volume fails, I'd probably 
> lose everything stored in that volume?!?  Sounds like a somewhat risky 
> business to me, unless you *really* needed a storage volume that big 
> that you had to span multiple drives to do so.

That is correct. You can add more drives and expand the volume group in
order to increase the size of your logical volumes. Losing one drive
would indeed cause all your data in the volume group to be lost
(although there are some situations where it isn't too difficult to
recover some of the data). 

What I think most people do (and what I am doing now), is to setup
RAID-1 or so behind the volume group. This way you will still be safe if
one of the drives fail. Keep in mind that RAID is not a backup solution,
and you should still create regular backups. :)

> > Seperating them will mean that if your OS drive fails you can replace
> > the dead drive, reinstall CentOS (or restore from a backup), and your
> > data will be accessible again.
> >   
> Kind of what I had in mind, as the 13GB drive is much older (circa 2005, 
> if its the original one put in when the previous owner built the box 
> from a bare-bones kit) than the 500GB SATA drive (earlier this year, 
> when I stuck it in there) so if I had to put money on one failing before 
> the other... ;)
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M. Hamzah Khan
RedHat Certified Engineer Number: 804005539516829
Email: hamzah at hamzahkhan.com
URL: http://www.hamzahkhan.com
Mobile: +44 (0)7525663951
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