[Bug libc/18515] posix_fallocate disastrous fallback behavior is no longer mandated by POSIX and should be fixed

["carlos at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18515

--- Comment #3 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Florian Weimer from comment #2)
> (In reply to Pat from comment #0)
> > The fallback behavior of posix_fallocate() is a disaster. The entire point
> > of this function is to improve system performance by allowing applications
> > to preallocate large extents, helping the O/S to minimize on-disk
> > fragmentation.
> > 
> > But the behavior of this call on file systems lacking unwritten extents
> > ultimately results in two writes to every "preallocated" block: Once to
> > "preallocate", and once to populate with actual data. This is the exact
> > opposite of a performance improvement.
> 
> Callers who want to avoid double-writes can use fallocate instead.  This is
> explained in the glibc manual.
> 
> We analyzed this issue recently, and we decided that we have to preserve the
> fallback (note that I completely changed my opinion after reviewing how
> application code uses posix_fallocate right now):
> 
> https://sourceware.org/ml/libc-alpha/2015-04/msg00309.html
> https://sourceware.org/ml/libc-alpha/2015-05/msg00058.html
> 
> So we really can't fix this bug, even though fallback is problematic.

Agreed. I'll submit an update to the linux kernel man pages to mention this
there also.

c.

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