On Sat, 2006-03-04 at 19:05 -0800, Andrew Morton wrote:
> [email protected] (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 <[email protected]>
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