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bug#14592: dd - possible error

From: Jared Still
Subject: bug#14592: dd - possible error
Date: Tue, 11 Jun 2013 14:36:00 -0700

Investigating an error in backup script I find that the use of dd parameter
iflag=direct is giving incorrect results in some circumstances.

address@hidden> dd --version
dd (coreutils) 6.3
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software.  You may redistribute copies of it under the terms of
the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
There is NO WARRANTY, to the extent permitted by law.

Written by Paul Rubin, David MacKenzie, and Stuart Kemp.

I realize this is an old version, but there is little I can do about that.

The situation:
A file is created and the md5sum returned:

origsum=`dd bs=1048576 if=source_file 2>/dev/null| tee destfile | md5sum `
destsum=`dd if=destfile bs=1048576 iflag=direct 2>/dev/null| md5sum

Source and Destination are both NFS V3, separate mount points.

When used with iflag=direct on the destination file, the md5sums do not
When iflag=direct is removed, the md5sum is correct.

This may or may not be an issue with dd.

Have there been any bugs reported for something similar to this?
I have not ruled out other issues, nfs, etc, but this so far has only been
seen with dd.

This was also duplicated without md5sum in the command line.
That is, local files were created with and without iflag=direct.

The md5sums do not match these files, with the same sums as seen when piped
to md5sum.

A hex dump of each file was created, and a diff taken.

Here is a sample of the diffs if it is of any value:

< 217134000 9d00 0000 3205 0400 36c2 0055 c305 4c3b
> 217134000 9d00 0000 c305 543b 0032 c204 5536 0500
-- -------------------
< 217134020 001d c204 5536 0800 7078 0c07 0111 0001
> 217134020 3bc3 1d4c 0400 36c2 0055 7808 0770 110c
-- -------------------
< 217134040 3402 0600 8110 32c3 6e00 0000 c305 543b
> 217134040 0101 0200 0034 1006 c381 0032 006e 0500
-- -------------------
< 217134060 0032 c204 5536 0500 3bc3 1d4c 0400 36c2
> 217134060 3bc3 3254 0400 36c2 0055 c305 4c3b 001d

Thanks for reading this far,

*Jared Still*
Team Technical Lead

Pythian - Love your data

address@hidden | Twitter: @PerlDBA | Linkedin:

Tel: +1 866 798 4426 x1267



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