GNU bug report logs - #12626
Bug?: dd limited to <2G read size (2G-8K) on 64 bit machine?

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Message #19 received at 12626-done <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: Linda Walsh <coreutils <at> tlinx.org>, 12626-done <at> debbugs.gnu.org
Subject: Re: bug#12626: Bug?: dd limited to <2G read size (2G-8K) on 64 bit
	machine?
Date: Fri, 12 Oct 2012 10:14:13 +0100

tag 12626 notabug
stop

On 10/12/2012 07:42 AM, Paul Eggert wrote:
> On 10/11/2012 11:39 PM, Linda Walsh wrote:
>> I'm not even getting error messages if I try to do 4G @ a time
>
> That's what dd is supposed to do.  It requests 4 GB,
> the kernel gives it only 2 GB, so it goes with 2 GB.
> dd has always worked that way, and POSIX requires it
> to work that way.

As my mother would say, that's a lazy man's load.
Please use a more reasonable buffer size.

Also note the iflag=fullblock option, which
will ensure the internal buffer is filled.
Then count= will refer to the number of buffers
processed, rather than the number of read()s.

Also note if you specify ibs and obs seperately,
then you will get writes of the size you requested at least.
But be careful with using this behavior in conjunction
with count=, as that will limit the number of read()s.

cheers,
Pádraig.

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