The policy that you use to enforce a given quota on a system.

I am thinking about disk quota, the amount of space that a user has on a given filesystem.
The typical Unix approach is to let you write stuff until you attempt to write over quota; in that case the write operation fails. This is generally enough for system that have reasonably smart technical users.

One day, I had to design a quota policy for a system that is basically a giant POP toaster. We decided for 10 megs of space per user.
And then we had a problem: should we reject a piece of mail that would have filled the quota ? For example, if the user is a 9 megs and a 2 megs attachment arrives, what do we do ?
If you reject it, then following smaller messages may actually end up in the mailbox. The unwary user would be left with a mysterious undelivered message.
So we decided that (this is a secret !) the real quota is 11 megs (N-Wing suggests that this should be called "the limit"). And we accept no message that overspills the mailbox.

When a user arrives at 9 megs, he receives a message (at most one per day) that encourages him to clean up his mailbox.
If the user stays between 9 and 10 megs forever, he keeps getting annoyingly polite messages forever.
When a user goes beyond 10 megs, he gets exactly one day warning before we start throwing away his old mail. We throw away enough old mail to reach 7 megs.

An interesting fact is that the great majority of the users, in fact, keep their mailboxes at less than one megabyte. Of course, when you start having thousands of users even a small minority may occupy various gigabytes.

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