On Wed, Dec 01, 2010 at 04:38:00PM +0000, Hugo Mills wrote: > On Wed, Dec 01, 2010 at 09:21:36AM -0500, Josef Bacik wrote: > > === Quotas === > > > > This is a huge topic in and of itself, but Christoph mentioned wanting to have > > an idea of what we wanted to do with it, so I'm putting it here. There are > > really 2 things here > > > > 1) Limiting the size of subvolumes. This is really easy for us, just create a > > subvolume and at creation time set a maximum size it can grow to and not let it > > go farther than that. Nice, simple and straightforward. > > > > 2) Normal quotas, via the quota tools. This just comes down to how do we want > > to charge users, do we want to do it per subvolume, or per filesystem. My vote > > is per filesystem. Obviously this will make it tricky with snapshots, but I > > think if we're just charging the diff's between the original volume and the > > snapshot to the user then that will be the easiest for people to understand, > > rather than making a snapshot all of a sudden count the users currently used > > quota * 2. > > This is going to be tricky to get the semantics right, I suspect. > > Say you've created a subvolume, A, containing 10G of Useful Stuff > (say, a base image for VMs). This counts 10G against your quota. Now, > I come along and snapshot that subvolume (as a writable subvolume) -- > call it B. This is essentially free for me, because I've got a COW > copy of your subvolume (and the original counts against your quota). > > If I now modify a file in subvolume B, the full modified section > goes onto my quota. This is all well and good. But what happens if you > delete your subvolume, A? Suddenly, I get lumbered with 10G of extra > files. Worse, what happens if someone else had made a snapshot of A, > too? Who gets the 10G added to their quota, me or them? What if I'd > filled up my quota? Would that stop you from deleting your copy, > because my copy can't be charged against my quota? Would I just end up > unexpectedly 10G over quota? > > This is a whole gigantic can of worms, as far as I can see, and I > don't think it's going to be possible to implement quotas, even on a > filesystem level, until there's some good and functional model for > dealing with all the implications of COW copies. :( In your case, it would sound fair that everyone is "simply" charged 10G. What Josef is refering to would probably only apply to volumes and snapshots owned by the same user: If I have a subvolume of 10G, and a snapshot of it where I only changed 1G, the charged quota would be 11G, not 20G. Mike -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
