Re: posix_fallocate question again

[Nikita Danilov]

Andrew Morton writes:
 > "Chen, Kenneth W" <kenneth.w.chen@xxxxxxxxx> 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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