tar+/dev/null weirdness

T. Horsnell tsh at mrc-lmb.cam.ac.uk
Thu May 5 12:32:31 UTC 2005


>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.


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