The thing to note is, when you make a clone, you are validating the
original backup. This could be priceless.
Bob
-----Original Message-----
From: George Sinclair [mailto:George.Sinclair AT NOAA DOT GOV]
Sent: 17 April 2003 14:59
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Subject: [Networker] Merits of cloning versus dual backups?
I've been messing around with cloning, and I would like to hear some
feedback, criticism and advice about its merits as opposed to just
creating a second backup tape. Let's suppose I have to create a one-time
backup of some static data, and I want to have two tapes in case I loose
one or one goes bad. Obviously, I could set up cloning to have the
savesets cloned to another tape as I back them up to the first.
Alternatively, I could just maintain two separate backup tapes, maybe
each being a member of a different pool like full and full2 where
anything on a tape in the full2 pool is a second backup of its
counterpart on the full pool. In thinking about this, I just don't see
how cloning would be the preferred method for the following reasons:
1. Cloning is slow because it must wait until the backups are finished
on the source tape before it can begin cloning these savesets to the
clone volume. Also, it cannot multi-plex the savesets to the clone
volume. Instead, it must write them one at a time. That could be along
haul!!!
On the other hand, if you just make two backups, there is nothing to
preclude you from having the data multi-plexed to the second tape just
like the first; so, it's way faster and you're not tying up that second
drive anywhere near as long as cloning would.
2. Maybe I'm wrong about this one, but I thought I read that if you
remove the savesets on the clone you wipe out the counterpart saveset on
the original since they both share the same SSID. Vice versa, too.
This is a moot point with two backup volumes as the same savesets would
have different SSIDs, making them independent of each other. You've
never have to worry about some accidentally setting off a chain
deletion.
3. For a one-time backup, cloning would have the advantage of not
listing the same saveset under the saveset recover window more than
once, whereas making a separate backup tape would cause the same saveset
to be listed twice -- could cause confusion -- but given the fact that
we're talking about static data, it would be obvious that one was a copy
since they'd both have the same size and 'details' would show the
volumes named something meaningful like:
full.001
full2.001
So you'd know that the one on say full2.001 was most likely a duplicate
backup.
I guess the only real advantage to cloning that I see is that it affords
you the ability to create a second volume in the event that the savesets
on the original volume can no longer be re-created because they no
longer exist on disk. So, you can't just go back, for example, and
re-back them up a second time. That might not be an option. Furthermore,
even if you could go back and re-back them up, they might have long
since changed, leaving the second backups of these savesets possibly way
different than the first. I guess it all depends on how long after the
fact you decided to create that second backup tape, but assuming we're
talking about static data, I really just don't see why cloning would be
preferred. I think making a second backup tape in close time proximity
to the first would be the fastest and most expeditious way to go -- you
have multi-plexing working for you for goodness sakes -- not to mention
the fact that you don't have to mess with the original tape while you're
doing it.
How do you guys deal with the slowness?! I mean if a bunch of savesets
are being backed up to tape, and then NetWorker has to come along later
that night, or whenever, and clone those, it's gonna grind through each
one one at a time, right? I mean, it can't very well clone more than one
at a time since it can only read one at a time. Of course, you could
clone the entire volume, but that goes one at a time, too, and it would
make no sense if the original source tape wasn't full yet. I just don't
see why anyone would not instead opt to just create dual backups to
separate pools as they go. Can someone convince me otherwise, please?
I'd love to hear that I'm wrong cause I think I might be, and I don't
want to be ignorant longer than necessary.
Sorry to be so long winded.
Thanks.
George
George.Sinclair AT noaa DOT gov
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