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The problem here is that the total size is somewhat unpredictable, from an administrative point of view.
I don't agree, though of course I don't work there.

However, just looking at it, you would still have the same amount of predictability with COW since each user would still be allocated his/her allotment of disk drive and you would know how much each distro takes up.
In fact, you would considerably save on harddrive space since most users would then never use their full potential of space. In theory you could over subscribe your harddrive space and just add to your raid when it starts to get full.
This obviously benefits users as well since now they've got a bunch more space to work with. Nice for power users, etc. It would give you a whole lot more room to offer various packages as well (offer 200MB, 500MB, 1GB, 1.5GB, etc.).
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Secondly, the backing files aren't easily migratable -- the COW files themselves won't mount if you don't have the exact backing file that was used when it was "created".
Yes, but again, this should never be a problem. I mean, once a virtual environment is created, you don't go in and change the user's base distribution anyway. You would leave it up to the end user to do so. (ie. applying patches, errata, etc. to their distro).
With COW, yes you would have to maintain all the backing files pretty much forever (or until no more customers used them), but I can't imagine this being that big of a deal.
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The only advantage I've seen to COW would be a "snapshot" feature which you could restore to in a second.
Yes, again another very strong point to using it.
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I just felt better about making things more simple. Less to go wrong with COWs and whatnot.
Yes, it's like a bit easier to deal with in the way you describe.
I'm not flaming here or anything. I'm just playing a bit of devil's advocate. Mostly, I want the harddrive space. I think your harddrive pricing is a bit higher than other quality virtual providers. I like what you're doing and I like the usermode linux project _a lot_. But, without COW, I'd have to think hard about jumping on. With in, I'd probably give you my credit card today.
Thanks again for discussion.