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    SubjectRe: Deleting large files

    On Wed, 7 May 2008, Morten Welinder wrote:

    > Hi there,
    > deleting large files, say on the order of 4.6GB, takes approximately forever.
    > Why is that? Well, it is because a lot of things need to take place to free
    > the formerly used space, but my real question is "why does the unlink caller
    > have to wait for it?"
    > I.e., could unlink do the directory changes and then hand off the rest of the
    > task to a kernel thread?
    > Morten
    > --
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    > Please read the FAQ at

    Suppose you had an N GB file that just filled up the disk. You now
    delete it, but get control back before it is really deleted. You
    now start to write a new file that will eventually just fill up
    the disk. Your task will get a media full error long before
    the media is really full because the old file's data space
    hasn't been freed yet. So, to "fix" this, you modify the file-
    system to defer your logical writes until all the previous
    spaces has been freed (writes to the physical media are deferred
    anyway as long as there is RAM available). The result is that
    your new data, that may be precious from a quasi-real-time source,
    will fail to be written. To "fix" this, you queue everything.
    This will eventually fail because the disk and RAM are of
    a finite size. The size of the disk is known, but you don't
    know what will be deleted before the queued writes have
    completed, so you really don't know when to tell the writer
    that there is no more space available.

    That's why the task that deletes data can't get control back
    until is has been deleted. However, for user applications, at
    the user's risk, one can do `rm filename &` and let the shell
    do the waiting.

    Dick Johnson
    Penguin : Linux version on an i686 machine (5588.29 BogoMips).
    My book :

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