[Gloucs] ext4 file modification times too precise

[Andrew M.A. Cater]

On Tue, Jun 29, 2010 at 07:26:22PM +0100, Paul Broadhead wrote:
> Hi folks,
> 
> I recently reinstalled Ubuntu 10.04 as I have a new computer:) The
> machine is now using ext4 file systems where the old machine used ext3
> and the same OS.
> 
> I use a simple backup script which results in some tar.gz files being
> copied to my server using a "cp -puvRf $source/* .", the important bit
> is the -up options so that only changed files get copied, and copies
> preserve the attributes of the original. The server, running Debian
> 5.0.5, is exporting the destination file system which is using ext3.
> 
> The problem is that it appears ext4 stores the modification time to a
> larger precision than ext3, so for the copied files, the original
> ext4 located file may have a stat modification time say:
> Modify: 2010-06-27 07:57:27.622601087 +0100
> 
> The copy on the ext3 file system has:
> Modify: 2010-06-27 07:57:27.000000000 +0100
> 
> This means that the -u part of the cp command thinks the original file
> is always newer than the copy.  This means that each time by backup
> runs, it copies over every file causing a lot of unnecessary network
> traffic.
> 
> So having failed with google, is there a way to fix this without
> changing file systems? Possibly with mount or export options? I have
> added a hack to the backup script to touch the original file with the
> time stamp from the copy. This fixes the problem but it is a hack!
> -- 
> 
> Regards,
> Paul
> 
> 
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Try rsync -pavz rather than using cp ?

Andy