tar+/dev/null weirdness

[T. Horsnell]

>>On Wed, 2005-05-04 at 12:36, T. Horsnell wrote:
>>
>>> I'm trying to investigate tar speeds over NFS. I normally test
>>> tar speeds by 'tar cf /dev/null somefilesystem' but if the filesystem
>>> is an NFS one, using /dev/null as the output file is resulting in
>>> impossible tar speeds. If the tar output is directed to a file
>>> rather than to /dev/null, all is well. Does tar do some sort
>>> of magic if its tarring NFS stuff to /dev/null? For example:
>>
>>GNU tar recognizes /dev/null as something magic whether or not
>>the files are on NFS and it skips reading the actual data.  This
>>feature is used by the amanda backup program to quickly obtain
>>estimates of full vs. incremental backup sizes.  If you want
>>to force the files to be read, try 'tar cf - somedir >/dev/null'
>>instead.
>
>Aha. Many thanks!
>T.

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...

Cheers,
Terry.

>
>
>>
>>-- 
>>  Les Mikesell
>>   les at futuresource.com
>>
>>
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