Re: [gentoo-user] LVM : pros & cons

[Alan McKinnon]

On Sunday 07 October 2007, Philip Webb wrote:
> Does anyone have advice based on experience using LVM ?

The majority of folks around here will have used LVM :-)

> I sb partitioning a new  320 GB  hard drive soon for a simple desktop
> box. That is  8 times  the size of the HDD in my present machine,
> which I haven't exhausted by any means.  LVM seems more professional
> & allows flexibility for unforeseen storage needs,

this is it's main benefit on desktop class machines - the ability to 
resize volumes when you realize that you guessed wrong. There are 
others too :-)

> but it adds a layer of complexity & potential problems arising
> therefrom.

A total non-issue in my experience. I've never had an LVM problem yet, 
but maybe I'm just lucky. The one thing you do need to be aware of it 
that you require LVM support at boot time or shortly thereafter. So 
either compile it into the kernel, or make sure it's in the initrd.

For a gentoo system using roll-your-own kernels, the consensus seems to 
be a regular / volume of 500M-1G is plenty and everything else is on 
LVM. That way you avoid the issues of not having the required support 
to be able to mount /. We don't build distro kernels that must boot on 
everything out there, we have the luxury of customizing everything

> I wonder whether LVM slows down disk access

No. See my other mail.

> & whether there's a disaster lurking unseen if anything goes wrong
> with LVM: a bad package update, a damaged config file or file storing
> LVM's layout would seem to risk losing everything on the HDD &
> require re-installation.

Not true. You *already* have many layers of software between user space 
and disk, any one of which can go wrong at any time. LVM metadata is 
stored in text files and it maintains many historical copies of 
previous configs and it's easy to fix if it ever goes wrong. I've never 
seen a *real* LVM error, but I have matched myself do some really dumb 
things and I could fix them every time.

Seriously, the problem LVM solves has been known about for decades and 
the method used was worked out about the same time. It's a mature 
technology that is *very* well understood, completely the opposite of 
drivers to support some latest new-fangled chipset. I would be much 
more worried about that code trashing your disk than LVM. Just to put 
it all into perspective...

Extra benefits of LVM: You won't need this right now for your simple 
desktop with one drive, but it's good to know what else LVM can do:

Snapshots. You can "freeze" the state of a filesystem at any time and 
LVM will track the changes since then until you release the snapshot. 
This is a lifesaver if your job is to perform backups of 4TB databases 
that can never be taken down for backups.

Huge volumes: LVM is the only way on Linux to be able to get local 
volumes bigger than any single disk. Again, on servers, 2TB+ databases 
are becoming commonplace.

If you need any more convincing, IBM mainframes and HP machines running 
HP-UX have required you to use LVM for years now - you can't get to the 
disks without using LVM. If it was risky, do you think those hardware 
vendors would have gone down that route?

alan




-- 
Optimists say the glass is half full,
Pessimists say the glass is half empty,
Developers say wtf is the glass twice as big as it needs to be?

Alan McKinnon
alan at linuxholdings dot co dot za
+27 82, double three seven, one nine three five
-- 
gentoo-user@g.o mailing list