GNU bug report logs - #13135
Loss of data while copying

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Eric Blake <eblake <at> redhat.com>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#13135: closed (Loss of data while copying)
Date: Mon, 10 Dec 2012 15:11:02 +0000

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Your message dated Mon, 10 Dec 2012 08:09:44 -0700
with message-id <50C5FB38.6040209 <at> redhat.com>
and subject line Re: bug#13135: Loss of data while copying
has caused the debbugs.gnu.org bug report #13135,
regarding Loss of data while copying
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
13135: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13135
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Cojocaru Alexandru <xojoc <at> gmx.com>
To: bug-coreutils <at> gnu.org
Subject: Loss of data while copying
Date: Mon, 10 Dec 2012 15:58:48 +0100

bash$ yes $(for i in $(seq 1 100000); do echo -n a; done) | dd of=big-lines ibs=100001 count=10000
9924+76 records in
1951183+1 records out
*999005921* bytes (999 MB) copied, 21.6135 s, 46.2 MB/s

bash$ yes $(for i in $(seq 1 100000); do echo -n a; done) | dd of=big-lines ibs=100001 count=10000
9887+113 records in
1950792+1 records out
*998805894* bytes (999 MB) copied, 21.3409 s, 46.8 MB/s

bash$ yes $(for i in $(seq 1 100000); do echo -n a; done) | dd of=big-lines ibs=100001 count=10000
9890+110 records in
1950792+1 records out
*998805919* bytes (999 MB) copied, 21.5801 s, 46.3 MB/s

bash$ yes $(for i in $(seq 1 100000); do echo -n a; done) | dd of=big-lines ibs=100001 count=10000
9884+116 records in
1950011+1 records out
*998405915* bytes (998 MB) copied, 25.2695 s, 39.5 MB/s

WTF?!

Best regards,
Cojocaru Alexandru

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From: Eric Blake <eblake <at> redhat.com>
To: xojoc <at> gmx.com
Cc: 13135-done <at> debbugs.gnu.org
Subject: Re: bug#13135: Loss of data while copying
Date: Mon, 10 Dec 2012 08:09:44 -0700

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tag 13135 notabug
thanks

On 12/10/2012 07:58 AM, Cojocaru Alexandru wrote:
> bash$ yes $(for i in $(seq 1 100000); do echo -n a; done) | dd of=big-lines ibs=100001 count=10000
> 9924+76 records in

Thanks for the report.  Based on this output, short reads occurred.  dd
transferred exactly 10000 reads as requested, but since some of those
were short, it transferred less than 10000*100001 bytes.  This is
expected (and the behavior is described in that way by POSIX); the
solution you are looking for is to _also_ use the iconv=fullblock
option, to force dd to re-read until it has a full input block rather
than immediately transferring short input reads to output.

As such, I'm closing this as not a bug.  Do feel free to add further
comments to this thread, though, if you have more questions about why dd
does this.

Oh, and by the way, 'echo -n' is not portable.  You want to use
printf(1) instead.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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