Re: [PATCH 0/4] Fiemap, an extent mapping ioctl - round 2

[Jamie Lokier <jamie@shareable.org>]

jim owens wrote:
> Dave Chinner wrote:
> 
> >The point of this SYNC flag is to ensure that you get nothing other
> >than blocks mapped to disk - no delalloc regions, etc. The only sane
> >way to do that is an atomic 'sync+map' operation. This is not a
> >filesystem specific feature - it's what the SYNC flag should be
> >defined as providing.
> 
> If the real need is to force allocation then the flag should
> be something like FIEMAP_FLAG_ALLOC and not need to do fsync
> or any data flush, just ensure there is assigned storage.

See also the huge time for fsync on ext3 in some circumstances (and
why they had to patch Firefox 3 because of it).  On ext3,
FIEMAP_FLAG_ALLOC would be a no-op, but sync can take a long time.

If the utilities using FIEMAP just need "no delalloc extents", they
really should use an ALLOC flag, if only because forcing writeback,
which may take a long time, is not what they are trying to accomplish.

-- Jamie