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Re: [Bug-tar] --listed-incremental ignores changed files when using file

From: Sergey Poznyakoff
Subject: Re: [Bug-tar] --listed-incremental ignores changed files when using filesystem snapshots
Date: Sun, 15 Nov 2009 12:15:21 +0200

Hi Bdale,

> When creating incremental backups using snapshot filesystems, tar fails
> to include files in a level 1 backup (presumably any level > 0) if they
> were modified in the original filesystem between the time the snapshot
> was created and the time the level 0 backup was created of the
> snapshot.

Yes, it is a known fact. There is a certain lapse of time between a
file is registered in the incremental snapshot and actually archived.
If it changes in between, these changes are not stored in the snapshot.

> I have had a look at the source, and the cause seems to be that the option
> newer_mtime_option is set from the creation time of the previous incremental
> archive (in incremen.c:read_incr_db_01 and incremen.c:read_incr_db_2)
> While I can't think of any good reason to set this option at all, except as a
> (failed) attempt to avoid unnecessary work, I cannot assess the full
> impact of simply removing the code in question.

The full impact of this would be rendering the incremental backup code
completely inoperational and useless: tar's incremental backups are
based on comparing file's mtimes with the reference timestamp stored in
the snapshot file.

Right now I cannot propose any immediate solution to this.


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