quota behaviour of reflink()

Posted May 6, 2009 11:21 UTC (Wed) by pjm (guest, #2080)
In reply to: The two sides of reflink() by amw
Parent article: The two sides of reflink()

One issue is what happens when one of the copies is removed, especially if there's more than one owner involved. Presumably the result must be that removing a file can increase someone else's quota usage. One wonders then what should happen if that other user has already exhausted their quota, and what bugs may be triggered by not expecting whichever policy is chosen.

The reasons for different quota behaviour would be if it results in different (and more desirable) user behaviour, or if it helps system administrators choose better quota limits, i.e. if it results in less frequent filesystem-full situations for a given amount of user productivity.

How much are quotas used these days, and for what uses ? Can people comment on the usefulness of different quota policies in the context of specific use cases?

As to whether different quota behaviour would result in different user behaviour (e.g. encouraging taking steps for files to be reflink'ed rather than copied), I wonder how many quota'd users would have the necessary knowledge for it to change their behaviour.

quota behaviour of reflink()

Posted May 6, 2009 12:47 UTC (Wed) by utoddl (guest, #1232) [Link]

Interesting points, but remember: quota can be impacted by unrelated actions by other processes owned by the same or other users at any time, so user space needs to respond to space-related issues regardless of what quotas existed "moments ago". In fact, quota can be affected when the user takes no actions at all; the admin can change quotas, file systems can be resized, etc., and a user who was not over quota may suddenly be so even with no changes to his files.

Space-reporting tools can report variations of (1) actual space used by extant allocated blocks (modulo sparse files), (2) space free, (3) space that would be used in the case where a naive copy were made to another file system -- all of which are valid and different numbers. The "simple" question of how much storage is used is in fact complicated, our desire for simple answers notwithstanding.