Re: tar incremental backups and ctime‏ problem

[Vince Rice <vrice@solidrocksystems.com>]

> On Mon, May 23, 2016 at 5:29 PM, Eric Blake <eblake@redhat.com> wrote:
>> On 05/23/2016 03:18 AM, x y wrote:
>>> It is not clear to me your expectation:
>>> - are you asking how to use ctime to select the file with tar alone ?
>>>  It is not possible for my understanding of the manual.
>>> 
>>> - Are you asking the package maintainer to change the behaviour of
>>>  cygwin tar ? Unlikely to happen, but I leave to him.
>>> 
>>> 
>>> Hi Marco,
>>> 
>>> Sorry, I am new to the mailing list. If I am not wrong, tar is
>>> checking both of the ctime and mtime values to compare files during
>>> incremental backups. Since opening and closing a MS document without
>>> changing the content updates ctime, it would be preferable to add a
>>> new option to tar to use only mtime for file comparing during
>>> incremental backups.
>> 
>> mtime is fakeable, ctime is not.  Using only mtime makes it likely that
>> your incremental backup will miss files.  I don't have any good reason
>> to differ from upstream behavior here.
> 
> Hi Eric,
> 
> The problem is not faking time stamps. Even commercial Windows backup
> programs are checking the modification time to identify the modified
> files.
> 
> Consider that you have a lot of files opened and closed without any
> modification in your company. Because of the priority of the ctime
> time stamp, reintroducing all of those files to the incremental backup
> does not make any sense. tar has also the capacity to create
> differential backups with the condition of taking care of the snapshot
> file. The ctime issue can result in unnecessarily big differential
> backups filled with unmodified files.
> 
> Cygwin tar can be a good  alternative for Windows users to do
> differential \ incremental backups but the ctime problem must be
> solved.

This is ultimately Eric’s decision, but…

It doesn’t _have_ to be solved. If someone wants to use Cygwin tar as a backup, then that someone lives with the fact that tar uses ctime. The differential might be a little too big, but no actual harm is done. So what.

I personally don’t want to have to guess at tar’s behavior. I want to know it’s the same on Cygwin as elsewhere.


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