[CentOS] evaluating backup systems: rsync

Sat Jan 19 19:21:38 UTC 2013
John Hinton <webmaster at ew3d.com>

On 1/19/2013 1:28 PM, Nicolas Thierry-Mieg wrote:
> Reindl Harald wrote:
>> Am 19.01.2013 15:46, schrieb Nicolas Thierry-Mieg:
>>> M. Fioretti wrote:
>>>> On Fri, Jan 18, 2013 08:07:40 AM -0500, SilverTip257 wrote:
>>>>> if you really want to eliminate that data being transferred, I
>>>>> suppose you could do the extra work and rename the directory at the
>>>>> same time on the source and destination.  Not ideal in the least.
>>>> Not ideal indeed, but I'll probably do it that way next time that some
>>>> renaming like this happens on very large folders. I assume that after
>>>> that, I'd also have to launch rsync with the options that says to not
>>>> consider modification time.
>>> no I don't think you will, since the file modification times won't have
>>> changed.
>> and even if the did - who cares?
>> * rsync does not transfer unchanged data ever
>> * rsync will sync the times to them from the sources
>> * so have nearly zero network traffic
> Not true: if you change the modification time on a file, by default
> rsync will copy the whole file again.
> See man rsync:
> Rsync  finds  files that need to be transferred using a “quick check”
> algorithm (by default) that looks for files that have changed in size or
> in last-modified time.
> and yes I've tested this before posting  ;-)
> to avoid this you need to use --size-only .
Yet size only is not reliable. If for instance you have a simple text 
file with the word hellO and someone catches the typo and changes it to 
hello, the filesize doesn't change as near as I can see. Both show as 6 
using ls -al. Unless rsync uses a more granular check of filesize that I 
am not aware of? If this is the case, then someone could potentially 
edit a large document fixing numerous simple typos and wind up with the 
same filesize.

John Hinton
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