Re: SEEK_HOLE and SEEK_DATA support?

[Nicholas Miell <nmiell@comcast.net>]

On Sat, 2006-03-04 at 19:05 -0800, Andrew Morton wrote:
> jimd@starshine.org (Jim Dennis) wrote:
> >
> > 
> >  All,
> > 
> >  Has there been any thought about adding SEEK_HOLE and SEEK_DATA (*)
> >  support to Linux?  
> > 
> >  I ask primarily because of the interplay between 64-bit systems and
> >  things like /var/log/lastlog (which appears as a 1.2TiB file due to
> >  the nfsnobody UID of 4294967294).
> > 
> >  (I'm realize that adding support for these additional seek() flags
> >  wouldn't solve the problem ... archiving tools would still have to
> >  implement it.  And I can also hear the argument that Red Hat and other
> >  distributions should re-implement lastlog handling to use a more modern
> >  and efficient hashing/index format and perhaps that they should set
> >  nfsnobody to "-1" ... I'd be curious if those details are driven by
> >  some published standard or if they are artifacts of porting.  I'd also
> >  be curious what's happened with other 64-bit UNIX ports and whether
> >  this issue ever came up in Linux ports to the Alpha or other 64-bit
> >  processors).
> > 
> >  As a stray data point I just did a quick experiment and just doing
> >  a time cat /var/log/lastlog > /dev/null took about:
> > 
> >  36.33user 2453.99system 41:35.90elapsed 99%CPU 
> >     (0avgtext+0avgdata 0maxresident)k
> >     0inputs+0outputs (133major+15minor)pagefaults 0swaps
> > 
> > 
> >  On an otherwise idle 2GHz dual Opteron (yes, of course the extra
> >  CPU is wasted for this job), reading SCSI disk hanging off a Fusion MPT 
> >  controller.
> > 
> >  From what I hear our Networker processes pore over these NULs for about
> >  two hours any time someone fails to exclude /var/log/lastlog from their
> >  backup list.
> 
> This can already be solved in userspace (and I'm sure it already has been
> in some backup programs).  Use the FIBMAP ioctl() against the fd to find
> out whether a particular block in the file is actually instantiated.
> 
> Not all filesystems necessarily implement FIBMAP, so one would need to fall
> back to sucky mode if FIBMAP failed.
> 

FIBMAP is a privileged operation.

-- 
Nicholas Miell <nmiell@comcast.net>

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