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

From: Andreas Dilger
Subject: Re: [Bug-tar] Problem with gnu tar and ZFS on Linux
Date: Thu, 21 Jun 2018 10:53:02 -0600

On Jun 21, 2018, at 9:23 AM, Pieter Bowman <address@hidden> wrote:
> We have been using gnu tar with amanda to backup our ZFS filesystems
> on Solaris (both SPARC and x86) for more than 10 years.  We take a ZFS
> snapshot (destroying the previous snapshot if it exists), run amanda
> pointing to the snapshot directory.  We are in the process of
> replacing our old Solaris x86 file server with one running CentOS and
> ZFS.  Unfortunately, that project has now stalled because the same
> process that we've been using no longer works.  Every night we end up
> backing up the full filesystem (only three at the moment, but that's
> still hundreds of gigabytes).  I did add the --no-check-device switch,
> but that didn't help.
> Here is an example set of commands (I forgot to save the "Totals" line
> from the first run, but it's approximately the same as from the second
> run):
> # Do full backup, creating the list incremental file
> tar --create --file - --directory /export/home/gnu/.zfs/snapshot/AMANDA 
> --one-file-system --no-check-device --listed-incremental /tmp/gnu_0 
> --ignore-failed-read --totals . | cat > /dev/null
> # Wait a day, destroying and creating a new snapshot
> cp /tmp/gnu_0 /tmp/gnu_0.new
> tar --create --file - --directory /export/home/gnu/.zfs/snapshot/AMANDA 
> --one-file-system --no-check-device --listed-incremental /tmp/gnu_0.new 
> --ignore-failed-read --totals . | cat > /dev/null
> Total bytes written: 81944340480 (77GiB, 75MiB/s)
> While examining the two listed incremental files, the timestamp at the
> beginning is obviously changed, but then a bit into the file there are
> directories that are in a different order, though the contents don't
> seem to have changed.
> Any ideas on what I should be looking for to try to resolve this
> issue?  I know this is also affecting other people.

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.

Cheers, Andreas

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