Alec Ten Harmsel <a...@alectenharmsel.com> writes: > On Tue, Jan 19, 2016 at 10:56:21PM +0100, lee wrote: >> Alec Ten Harmsel <a...@alectenharmsel.com> writes: >> > >> > Depends on how the load is. Right now I have a 500GB HDD at work. I use >> > VirtualBox and vagrant for testing various software. Every VM in >> > VirtualBox gets a 50GB hard disk, and I generally have 7 or 8 at a time. >> > Add in all the other stuff on my system, which includes a 200GB dataset, >> > and the disk is overcommitted. Of course, none of the VirtualBox disks >> > use anywhere near 50GB. >> >> True, that's for testing when you do know that the disk space will not >> be used and have no trouble when it is. When you have the VMs in >> production and users (employees) using them, you don't know when they >> will run out of disk space and trouble ensues. > > Almost. Here is an equal example: I am an admin on an HPC cluster. We > have a shared Lustre filesystem that people store work files in while > they are running jobs. It has around 1PB of capacity. As strange as this > may sound, this filesystem is overcommitted (we have 20,000 cores, > that's only 52GB per core, not even close to enough for more than half a > year of data accumulation). Unused data is deleted after 90 days, which > is why it can be overcommitted.
Why do you need to overcommit in the first place when you don't need that much disk space anyway? And it only works because you "shrink" the disk space used by deleting data. > Extending this to a more realistic example without automatic data > deletion is trivial. Imagine you are a web hosting provider. You allow > each client unlimited disk space, so you're automatically overcommitted. > In the aggregate, even though one client may increase their usage > extremely quickly, total usage rises slowly, giving you more than enough > time to increase the storage capacity of whatever backing filesystem is > hosting their files. I'm a customer of such a provider that used to do that, and they stopped giving their customers unlimited disk space years ago. I guess they found out that they can't possibly keep up with the demand, at least not without charging more. >> > All Joost is saying is that most resources can be overcommitted, since >> > all the users will not be using all their resources at the same time. >> >> How do you overcommit disk space and then shrink the VMs automatically >> when disk usage gets lower again? >> > > Sorry, my previous example was bad, since the normal strategy is to > expand when necessary as far as I know. See above. Well, that's exactly the problem. Once a VM has grown, it won't shrink automatically, which soon breaks the overcommitment.
