[vox-tech] Xen + LVM usage

[Luke Crawford]

On Wed, 21 Jun 2006, Bill Broadley wrote:
>
> In the last talk I mentioned using LVM to save disk space with Xen.
>
> I didn't quite catch the speakers explanation of why this might be
> a problem.

That is because I don't know if it would be a problem, myself.    However, 
I have been burned (horribly disfigured)  by putting vnd (loopback) 
backing files on a filesystem with softupdates/journaling enabled, so my 
first reaction to a suggestion of disk trickery is "Run away!"

At first I thought what you were suggesting is doing the union in the 
dom0, at the backing file level.   Being as this is two big files, and ffs 
with softupdates is much better at containing corruption to just a few 
files than it is at preventing corruption within those files, the result 
would be most likely disastrous.

However, if you did the union at the domU level, so it saw 1 complete 
filesystem as the original, and another complete filesystem (or journal, 
or whatever it uses) as the delta, then it should work fine.

As we all keep saying,  disk space is cheap.  However, your idea has 
merit, especially when,  say,  attacking the Chinese or Indian market... I 
imagine the difference between $5/month and $2/month would be 
significant in that market.  Heck, even for my $5 images now, it would 
increase my margins and decrease my startup costs.  Good SCSI is $3-$5 GB 
new (that's $6-$10 per GB after raiding it... more after paying for 
enclosures and stuff)  Honestly, power isn't that much of an issue with 
disks... it's CPU that eats the power, but disks do consume rack space 
(especially if you use lots of small disks like I do)

though, my cost per ipv4 IP is around $0.70/month, and my system is 
largely not-automatic.  I'd have to make it much more automatic to be 
able to profit from those kinds of margins.

It also appears that your idea would reduce my setup costs.  Right now, it 
takes a good 5-10 minutes to create a Xen image, and if this is simply a 
kind of 'reverse snapshot'  the setup should be just about instant.

Hmm.  So it does bear looking into. I'm bringing up a new dom0 tonight, 
and I'm considering linux rather than NetBSD.   We will see.