[SOLVED] tar+/dev/null weirdness

[T. Horsnell]

>On Thu, 2005-05-05 at 07:58, T. Horsnell wrote:
>
>> Oops, I spoke too soon...
>> 
>> On my system:
>> 
>> [root at ls1 ~]$ df -k /testnfs
>> Filesystem           1K-blocks      Used Available Use% Mounted on
>> /dev/sdd1             35280616  11836988  21651476  36% /testnfs
>> [root at ls1 ~]$ time tar cf - -C /testnfs . > /dev/null
>> 0.074u 0.000s 0:00.07 100.0%    0+0k 0+0io 0pf+0w
>> [root at ls1 ~]$ time tar cf /dev/null -C /testnfs .
>> 0.075u 0.000s 0:00.07 100.0%    0+0k 0+0io 0pf+0w
>> 
>> [root at ls1 ~]$ df -k /nfs/server1/www
>> Filesystem           1K-blocks      Used Available Use% Mounted on
>> server1:/www          35278544  11923576  21562920  36% /nfs/server1/www
>> [root at ls1 ~]$ time tar cf - -C /nfs/server1/www . > /dev/null
>> 0.303u 1.739s 0:08.73 23.2%     0+0k 0+0io 0pf+0w
>> [root at ls1 ~]$ time tar cf /dev/null -C /nfs/server1/www .
>> 0.294u 1.333s 0:16.00 10.1%     0+0k 0+0io 0pf+0w
>> 
>> [root at ls1 ~]$ tar --version
>> tar (GNU tar) 1.14
>> Copyright (C) 2004 Free Software Foundation, Inc.
>> This program comes with NO WARRANTY, to the extent permitted by law.
>> You may redistribute it under the terms of the GNU General Public License;
>> see the file named COPYING for details.
>> Written by John Gilmore and Jay Fenlason.
>> 
>> Les, is this *meant* to work as you say, or is it something
>> which just happened to work (once upon a time)? If its meant
>> to work, perhaps I should contact the Gnutar maintainers...
>
>It's intended to let the --totals option work as quickly
>as possible, but now I remember it is even smarter.  It
>is comparing it's output fd inode to that of /dev/null
>so even the redirection triggers the 'directory only'
>mode.  If you look at your file access times you'll see
>that they weren't really read.  You have to do something
>like 'tar cf - . |cat >/dev/null' to make it actually
>read the files and throw away the data.

That cracked it. Thanks very much.

Terry.