Re: [Bug-tar] [patch v3] Bug / question in tar

[Joerg Schilling]

Tim Kientzle <address@hidden> wrote:

> I'm curious.  If someone types the following command:
>
>    tar cf /dev/null some files
>
> What do you think they expect to happen?

Of course that all files are read. POSIX does not allow different behavior just 
because the output is connected to a different sink.

Maybe this is why n-1 tar implenentations implement a uniform behavior.
Star implements a special documented option for the case that a user is 
interested in getting the size of the archive to be created quickly.
If one user (amanda) expects side-effects, I would see this as a bug.

tar cf /dev/null some files 

Is used by many people to do one of the following:

-       Do speed tests for the underlying filesystem

-       Give the OS the opportunity to cache data and meta data

-       Make sure that all data and meta data in a filesystem is read
        in order to trigger disks to do auto-reallocation on blocks
        that are going to become bad.

-       Make sure that all data and meta data in a filesystem is readable

Gtar does not allow this to be done easily and keep in mind that many OS 
implementations have a significant performance loss when using pipes.

Jörg

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