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Re: [Bug-tar] Problem with gnu tar and ZFS on Linux

From: Joerg Schilling
Subject: Re: [Bug-tar] Problem with gnu tar and ZFS on Linux
Date: Fri, 22 Jun 2018 10:49:13 +0200
User-agent: Heirloom mailx 12.5 7/5/10

Andreas Dilger <address@hidden> wrote:

> Keep your old snapshot around after you create the new one, and run "stat(1)"
> on the same file in both snapshots to see if there are any other differences
> besides the device number that may be causing tar to think the file changed.

gtar likes to implement something that is in conflict with a reliable 
filesystem backup. It tries to support directory trees without looking at mount 
point limits. This requires to look at both, stat.st_ino and stat.st_dev, since
inodes are unique only on a single filesystem.

Reliable filesystem backups on the other side need to be based on snapshots an 
snapshots need to create ephemeral st_dev values in order to create the 
appearance of a different filesystem with POSIX semantics.

This does not work well together...

Please have a look at star's incrementals. There are examples that are 
currently starting at page #55. Examples for incrementals are in the section 
INCREMENTAL BACKUPS that currently starts at page #50.


Note that star's incrementals are smaller than what you get from gtar:

If you have a full 1TB disk with a single directory tree in it's root directory
and rename the top level diretory just under the root, you end up with a 10 kB
star incremental.

The same done with gtar needs 1TB of backup space and you would even need a 
2 TB disk to be able to restore the incremental.

BTW: currently sourceforge again tells me since half an hour that it is under 
maintenence that will end in 10 minutes :-(

So be a bit patient when trying to access the man page.


 EMail:address@hidden                    (home) Jörg Schilling D-13353 Berlin
    address@hidden (work) Blog: http://schily.blogspot.com/
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