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Re: tar incremental backups and ctimeâ problem
- From: Vince Rice <vrice at solidrocksystems dot com>
- To: Cygwin Mailing List <cygwin at cygwin dot com>
- Date: Mon, 23 May 2016 10:24:07 -0500
- Subject: Re: tar incremental backups and ctimeâ problem
- Authentication-results: sourceware.org; auth=none
- References: <CAOKAGPP0o6vV2KTpfs4M0DyqwHA3pVw-+xkwLmjyWpAFthbA+A at mail dot gmail dot com> <9fdf98cf-e3d1-e453-1c98-2c206afe81c9 at gmail dot com> <CAOKAGPOzSYkmFRJazgQMwjUCKsKaBbE9gcVAdNnNvYUkmc=9Bw at mail dot gmail dot com> <09f604cd-61df-e0c7-b313-1dcf1ef59b4e at gmail dot com> <CAOKAGPMHEiNLW3makusvvx2E7V9etjA+YPUMDh56=N42wmtLYA at mail dot gmail dot com> <574313B3 dot 3090703 at redhat dot com> <CAOKAGPOVnU1Lr2od-UbtRAeZHFKjnNC2jOmmGouVotSYMxbokw at mail dot gmail dot 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 Ericâs decision, butâ
It doesnât _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 donât want to have to guess at tarâs behavior. I want to know itâs the same on Cygwin as elsewhere.
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