Re: FIBMAP/FIEMAP discrepancy for CAP_SYS_RAWIO

[Andreas Dilger <adilger@sun.com>]

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.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.