Why does tar appear to skip file contents when output file is /dev/null? The 2019 Stack...
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Why does tar appear to skip file contents when output file is /dev/null?
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Why does tar appear to skip file contents when output file is /dev/null?
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2019 Community Moderator Election ResultsWhy is my tar file bigger than its contents?mv a file to /dev/null breaks dev/nulltar: /dev/st0: Cannot write: Input/output errorMeaning of `cat /dev/null > file`Output file generated by tarWhy does my tar not work?What happens when you write to /dev/null? What’s the point?Why is /dev/null a file? Why isn't its function implemented as a simple program?Make tar from /dev/stdin fileCompare file compression methods by sending to /dev/null
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I have a directory with over 400 GiB of data in it. I wanted to check that all the files can be read without errors, so a simple way I thought of was to tar it into /dev/null. But instead I see the following behavior:
$ time tar cf /dev/null .
real 0m4.387s
user 0m3.462s
sys 0m0.185s
$ time tar cf - . > /dev/null
real 0m3.130s
user 0m3.091s
sys 0m0.035s
$ time tar cf - . | cat > /dev/null
^C
real 10m32.985s
user 0m1.942s
sys 0m33.764s
The third command above was forcibly stopped by Ctrl+C after having run for quite long already. Moreover, while the first two commands were working, activity indicator of the storage device containing . was nearly always idle. With the third command the indicator is constantly lit up, meaning extreme busyness.
So it seems that, when tar is able to find out that its output file is /dev/null, i.e. when /dev/null is directly opened to have the file handle which tar writes to, file body appears skipped. (Adding v option to tar does print all the files in the directory being tar'red.)
So I wonder, why is this so? Is it some kind of optimization? If yes, then why would tar even want to do such a dubious optimization for such a special case?
I'm using GNU tar 1.26 with glibc 2.27 on Linux 4.14.105 amd64.
tar null
asked 32 mins ago
RuslanRuslan
1,3961427
add a comment |
I have a directory with over 400 GiB of data in it. I wanted to check that all the files can be read without errors, so a simple way I thought of was to tar it into /dev/null. But instead I see the following behavior:
$ time tar cf /dev/null .
real 0m4.387s
user 0m3.462s
sys 0m0.185s
$ time tar cf - . > /dev/null
real 0m3.130s
user 0m3.091s
sys 0m0.035s
$ time tar cf - . | cat > /dev/null
^C
real 10m32.985s
user 0m1.942s
sys 0m33.764s
The third command above was forcibly stopped by Ctrl+C after having run for quite long already. Moreover, while the first two commands were working, activity indicator of the storage device containing . was nearly always idle. With the third command the indicator is constantly lit up, meaning extreme busyness.
So it seems that, when tar is able to find out that its output file is /dev/null, i.e. when /dev/null is directly opened to have the file handle which tar writes to, file body appears skipped. (Adding v option to tar does print all the files in the directory being tar'red.)
So I wonder, why is this so? Is it some kind of optimization? If yes, then why would tar even want to do such a dubious optimization for such a special case?
I'm using GNU tar 1.26 with glibc 2.27 on Linux 4.14.105 amd64.
tar null
asked 32 mins ago
RuslanRuslan
1,3961427
add a comment |
I have a directory with over 400 GiB of data in it. I wanted to check that all the files can be read without errors, so a simple way I thought of was to tar it into /dev/null. But instead I see the following behavior:
$ time tar cf /dev/null .
real 0m4.387s
user 0m3.462s
sys 0m0.185s
$ time tar cf - . > /dev/null
real 0m3.130s
user 0m3.091s
sys 0m0.035s
$ time tar cf - . | cat > /dev/null
^C
real 10m32.985s
user 0m1.942s
sys 0m33.764s
The third command above was forcibly stopped by Ctrl+C after having run for quite long already. Moreover, while the first two commands were working, activity indicator of the storage device containing . was nearly always idle. With the third command the indicator is constantly lit up, meaning extreme busyness.
So it seems that, when tar is able to find out that its output file is /dev/null, i.e. when /dev/null is directly opened to have the file handle which tar writes to, file body appears skipped. (Adding v option to tar does print all the files in the directory being tar'red.)
So I wonder, why is this so? Is it some kind of optimization? If yes, then why would tar even want to do such a dubious optimization for such a special case?
I'm using GNU tar 1.26 with glibc 2.27 on Linux 4.14.105 amd64.
tar null
asked 32 mins ago
RuslanRuslan
1,3961427
I have a directory with over 400 GiB of data in it. I wanted to check that all the files can be read without errors, so a simple way I thought of was to tar it into /dev/null. But instead I see the following behavior:
$ time tar cf /dev/null .
real 0m4.387s
user 0m3.462s
sys 0m0.185s
$ time tar cf - . > /dev/null
real 0m3.130s
user 0m3.091s
sys 0m0.035s
$ time tar cf - . | cat > /dev/null
^C
real 10m32.985s
user 0m1.942s
sys 0m33.764s
The third command above was forcibly stopped by Ctrl+C after having run for quite long already. Moreover, while the first two commands were working, activity indicator of the storage device containing . was nearly always idle. With the third command the indicator is constantly lit up, meaning extreme busyness.
So it seems that, when tar is able to find out that its output file is /dev/null, i.e. when /dev/null is directly opened to have the file handle which tar writes to, file body appears skipped. (Adding v option to tar does print all the files in the directory being tar'red.)
So I wonder, why is this so? Is it some kind of optimization? If yes, then why would tar even want to do such a dubious optimization for such a special case?
I'm using GNU tar 1.26 with glibc 2.27 on Linux 4.14.105 amd64.
tar null
tar null
asked 32 mins ago
RuslanRuslan
1,3961427
asked 32 mins ago
RuslanRuslan
1,3961427
asked 32 mins ago
RuslanRuslan
1,3961427
asked 32 mins ago
RuslanRuslan
1,3961427
asked 32 mins ago
RuslanRuslan
1,3961427
1,3961427
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
It is a documented optimization:
When the archive is being created to
/dev/null, GNU tar tries to
minimize input and output operations. The Amanda backup system, when
used with GNU tar, has an initial sizing pass which uses this feature.
answered 14 mins ago
murumuru
37.7k589165
add a comment |
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1 Answer
1
active
oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
It is a documented optimization:
When the archive is being created to
/dev/null, GNU tar tries to
minimize input and output operations. The Amanda backup system, when
used with GNU tar, has an initial sizing pass which uses this feature.
answered 14 mins ago
murumuru
37.7k589165
add a comment |
It is a documented optimization:
When the archive is being created to
/dev/null, GNU tar tries to
minimize input and output operations. The Amanda backup system, when
used with GNU tar, has an initial sizing pass which uses this feature.
answered 14 mins ago
murumuru
37.7k589165
add a comment |
It is a documented optimization:
When the archive is being created to
/dev/null, GNU tar tries to
minimize input and output operations. The Amanda backup system, when
used with GNU tar, has an initial sizing pass which uses this feature.
answered 14 mins ago
murumuru
37.7k589165
It is a documented optimization:
When the archive is being created to
/dev/null, GNU tar tries to
minimize input and output operations. The Amanda backup system, when
used with GNU tar, has an initial sizing pass which uses this feature.
answered 14 mins ago
murumuru
37.7k589165
answered 14 mins ago
murumuru
37.7k589165
answered 14 mins ago
murumuru
37.7k589165
answered 14 mins ago
murumuru
37.7k589165
37.7k589165
add a comment |
add a comment |
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