Re: FIBMAP/FIEMAP discrepancy for CAP_SYS_RAWIO

[Jamie Lokier <jamie@shareable.org>]

Andreas Dilger wrote:
> On Sep 07, 2009  16:26 +0000, Florian Weimer wrote:
> > * Andreas Dilger:
> > > If you are implementing a tool to use this, I would code it to try
> > > FIEMAP first, then FIBMAP (if it is running as root, or it gets
> > > fixed in some future kernel), then just do without (as it most likely
> > > does already today).
> > 
> > If FIBMAP is unsafe, it's likely exposed by concurrent changes to the
> > file, so using it would still be unsafe for backup purposes.  And I
> > really only need the number of the first block.  (I want to optimize
> > reading of Maildir-style folders, mainly for backup purposes.)
> 
> Given that the worst that can happen for your particular application
> if FIBMAP gets the wrong block number is a sub-optimal ordering for
> the file copy, there is no risk in doing this.
> 
> For the FIEMAP code, since you only need the first block number, just
> pass it a single fiemap_extent so that it doesn't spend time generating
> a full list of extents that you don't need to use.

With FIEMAP, filesystems which don't use extents will still scan a
potentially large region of the disk doing block lookups won't they,
just to maximise the size of the first extent in a large file?

Assuming yes, FIBMAP would be faster than FIEMAP in those cases, which
may be a reason to give it the same permissions, so it can be used.

-- Jamie