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Date: Mon, 23 May 2016 17:57:27 +0300
Message-ID: <CAOKAGPOVnU1Lr2od-UbtRAeZHFKjnNC2jOmmGouVotSYMxbokw@mail.gmail.com>
Subject: =?UTF-8?Q?Re=3A_tar_incremental_backups_and_ctime=E2=80=8F_problem?=
From: x y <x1y2w3 AT gmail DOT com>
To: cygwin AT cygwin DOT com
Content-Type: text/plain; charset=UTF-8

>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.



On Mon, May 23, 2016 at 5:29 PM, Eric Blake <eblake AT redhat DOT 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.
>>
>> Regards
>> Marco
>>
>>
>> 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.
>
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>

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