ZFS backups: retrieving a few files?

[Freddie Cash]

On Tue, Nov 23, 2010 at 4:56 AM, Thomas Ronner <thomas at ronner.org> wrote:
> On 11/23/10 1:45 PM, Andrew Reilly wrote:
>> No, I don't like tar, rsync and friends for backups: they don't
>> deal well with hard links, special files or sparse files.
>
> rsync -avHxS --delete --numeric-ids /src/. /dst/.
>
> Handles sparse files (S) and hard links (H). Never had any trouble with
> special files. What sort of special files are not handled correctly by
> rsync? I'd like to know because I'm relying on rsync for backups for years
> on my home network.

One problem with using rsync when dealing with hard-linked files:  it
doesn't like it when the source switches from hard-linked to
non-hard-linked files.  You end up with a mix of hard-linked and
non-hard-linked files in the destination, with the contents of the
non-hard-linked files all mixed around.

We just discovered this when we upgraded our Debian 4.0 (Etch) boxes
to Debian 5.0 (Lenny).  On Etch, all the gzip tools are hard-links
(zcat, zless, gzip, gunzip, etc).  On Lenny, they are all separate
files, and most are just shell scripts that use gzip.  Doing an rsync
of a Lenny box onto a directory from an Etch box, you end up with some
hard-linked files, some regular files, and the contents of all the
files are mixed-up based on which source file (script or binary) was
read first.

We've had to resort to clearing out the backups directory when doing a
Debian upgrade, in order to guarantee that we get a clean backup via
rsync.

-- 
Freddie Cash
fjwcash at gmail.com