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

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

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

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |fweimer at redhat dot com
         Resolution|---                         |WONTFIX
              Flags|                            |security-

--- Comment #2 from Florian Weimer <fweimer at redhat dot com> ---
(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.

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