GNU bug report logs - #8171
[dd] bs=blocksize doesn't work as expected

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Станислав
 <ginermail <at> gmail.com>
Subject: bug#8171: closed (Re: bug#8171: [dd] bs=blocksize doesn't work as
 expected)
Date: Fri, 04 Mar 2011 14:04:01 +0000

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Your bug report

#8171: [dd] bs=blocksize doesn't work as expected

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 8171 <at> debbugs.gnu.org.

-- 
8171: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8171
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Pádraig Brady <P <at> draigBrady.com>
To: Станислав <ginermail <at> gmail.com>
Cc: 8171-done <at> debbugs.gnu.org
Subject: Re: bug#8171: [dd] bs=blocksize doesn't work as expected
Date: Fri, 04 Mar 2011 14:02:35 +0000

On 04/03/11 05:02, Станислав wrote:
> [dd] bs=blocksize doesn't work as expected, for example:
> tar -c /somedir | dd bs=256K of=/dev/nst0
> In this example backup record will be recorded to a tape with block size =
> 10K instead of 256K because 10K is default for tar.
> There is workaround:
> tar -c /somedir | dd ibs=256K obs=256K of=/dev/nst0
> If we set block size for input and output separatly - it works as expected.

This operation is expected and dictated by POSIX.
It is surprising though, and I tried to clarify it in the docs:
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=d5ab0599

Note also that we're currently considering warning
that iflag=fullblock should be used, but only
currently in the case were a count is specified,
as then you get the wrong amount of data as well
as the wrong sized writes.
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=7362

cheers,
Pádraig.

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From: Станислав <ginermail <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: [dd] bs=blocksize doesn't work as expected
Date: Fri, 4 Mar 2011 08:02:56 +0300

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[dd] bs=blocksize doesn't work as expected, for example:
tar -c /somedir | dd bs=256K of=/dev/nst0
In this example backup record will be recorded to a tape with block size =
10K instead of 256K because 10K is default for tar.
There is workaround:
tar -c /somedir | dd ibs=256K obs=256K of=/dev/nst0
If we set block size for input and output separatly - it works as expected.

lucid <http://packages.ubuntu.com/lucid/coreutils> (utils): The GNU core
utilities
7.4-2ubuntu2: amd64 i386

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