[CentOS] OT how to prevent oversubscription of a disk

Tue Jan 4 19:01:30 UTC 2011
Benjamin Smith <lists at benjamindsmith.com>

On Monday, January 03, 2011 11:39:38 am Dave wrote:
> On Sat, Jan 1, 2011 at 10:06 PM, Gordon Messmer <yinyang at eburg.com> wrote:
> > On 01/01/2011 05:56 PM, Dave wrote:
> >> Is there a best practice? People have to be doing something!
> > 
> > I think that's unlikely.  If you don't "oversubscribe" your disk space
> > as a matter of policy, you'll force upgrades earlier than most people
> > would consider them necessary.  Most users, I'd expect, will be well
> > under quota most of the time.  You'd commit all of your disk space to
> > quota long before the space was actually used.  In your scenario, you'd
> > be required to expand the disk array whenever it was committed to quota,
> > even if actual use was very low.  Every site that I know of which uses
> > quotas handles disk upgrades when utilization requires it, not when
> > quota subscription does.
> So, is it fair to rephrase that as "ignore quotas, pay attention to
> actual usage"?
> I agree that some degree of oversubscription is probably desireable,
> and it would be much easier to just add storage whenever it looks to
> be getting fullish. My situation right now makes that difficult -
> budget is gone, so I can't add storage, and my users sometimes start
> up a big simulation that could potentially fill the disk right before
> the weekend. If the hoggy simulation crashes itself, that's okay, but
> if it brings down a lot of other jobs submitted by other users, I look
> bad. I guess even if there was some good tool support, this task is
> doomed to make everyone unhappy.

If you have no money for an upgrade, your hand is forced. You have several 

1) Do nothing, pray that your users don't exceed disk space available. If you 
have numerous customers and your average usage is far below quotas, this is 
likely to work. 

2) Change your TOS to account for the rare case where you actually run out of 
disk space. Sorta like number 1, but more honest. 

3) Reduce user quotas so you'd never overcommit. 

4) Offer a premium service to high-needs customers to help cover your costs. 

5) Dump your high needs customers and keep everybody else happy. 

That's pretty much it. (shrug) 

It's probably out of place for me to question your adminstrative decisions, 
but are we really talking about imposing limits to one of the cheapest things 
that there ARE in computer serviceland - disk space - at an average cost of 
about $70 per TERABYTE of commodity storage? Even if you went with SAS drives, 
the price only rises to about $150 per 750 GB - just how much space are your 
end users likely to need?

Maybe you "can't afford" to throw in another disk drive and mount as your /home 
directory, but more importantly, can you afford not to? 

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