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    SubjectRe: EXT2_UNRM_FL
    Theodore Y. Ts'o writes:
    > Date: Thu, 4 Mar 1999 11:00:45 -0500 (EST)
    > From: "Albert D. Cahalan" <>
    > There is another unrelated reason why the kernel must be involved.
    > You need to really delete files as the disk gets full. Polling is
    > just total crap, with race conditions and extra system load. You can
    > run out of disk space between two polls. That is not allowed.
    > Polling simply means doing a quick statfs; that doesn't take much system
    > load. And obviously, you want to keep a fairly large amount of free
    > space to make sure the filesystem can do good block allocation --- for
    > example, you might use a high and low watermark of 5% and 10% of the
    > filesystem size. So unless you the user processes can consume that much
    > disk space between the two polls, you'll be fine.
    > The grand Unix design transition is to try doing things in user space
    > first. If it doesn't work, then put in the minimal kernel hooks to
    > allow the Right Thing to happen. In this case, the smallest hooks would
    > be ones which can automatically wake up the daemon when filesystem space
    > starts getting critical.

    Assuming you're not suggesting calling the daemon synchronously, then
    there still is a delay between when the FS starts getting full, and
    when the daemon can start purging. The delay between this and when
    free space becomes available may be a significant fraction of a second
    (especially if a lot of small files need to be deleted to make room
    for a big one). So polling once or twice a second might not have any
    significant impact, anyway.

    > Also, please remember that there are plenty of user-space undelete
    > packages out there on the net; they apparently work just fine for
    > most people who have a desire for that kind of protection.

    And I still wonder whether you really need to bury this in unlink(3)
    rather than rm(1).



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