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SubjectRe: posix_fallocate question again
Andrew Morton writes:
> "Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:
> >
> > This has been brought up by Ulrich more than 3 years ago:
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=95569775802945&w=2
>
> A decent fallocate() implementation requires that the underlying fs has a
> permanent representation of blocks which are in an "allocated,
> uninitialised" state. afaik XFS is the only such filesystem.
>
> It's a fair bit of work for what doesn't really sound a very useful
> feature. Doing it in libc is reasonable. Probably the libc implementation
> could be improved by using ioctl(FIBMAP) and O_DIRECT to mimimise IO and
> CPU utilisation.

fallocate() will be useful when writing into file through
mmap(). Currently kernel can just drop dirtied page at any moment (if
->writepage() fails with -ENOSPC), so the only safe way to modify file
through mmap() is by using mlock().

>
> > Is there anytime soon that kernel 2.6 will have such functionality?
>
> Nope.
>

Nikita.

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