O.K., most people do hard in understanding the NW cloning
process and i have to admit that NW itself also does not
help in making understanding easy.
First, NW can clone as fast as it has backed up. And this is
a serious and true statement. Just assume that you have a
media with one only save set: If you have to clone it, this
is the scenario you will end up with. The drive's speed is
the bottleneck. It can be the source or the destination device, whatever is slower. Please keep in mind that you
copy from a media to a pool, so the clone media might be of
another type than the source media you used for backups.
However, most backup media will consist of multiplexed
(interleaved) save streams and this is what the situation
makes more complex. Can you also clone such volume in a
multiplexed way (in one pass), creating a multiplexed tape?
Yes, in general. And this is Networker's default way how to
to cloning. However, this rarely happens due to other
parameters in the scenario. The general problem here is
called "tape drive contention" or "the fight for drives".
Such usually happens if you have overlapping save groups
and you are using automatic cloning.
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On the other hand, in most cloning scenarios, you will not
clone the whole media, but only certain save sets. In this case, cloning will slow down because it will consolidate
the save sets (it will read the save sets sequentially while cloning). BTW, this is analog to a recovery process
where it is most unlikely that you will recover all
multiplexed save sets at the same time, especially not, if
they come from different clients. As sequential cloning
results in a decreased throughput (while de-multiplexing,
you must read through data that does not belong to this
certain save set) the cloning speed slows down and you need
more time.
Just think about an easy example:
- You are backing up 4 save sets of 10GB size equally
distributed. This results in 40GB of multiplexed data.
Let those be the only save sets on the backup media.
- If you do a volume clone, NW (if it can), will clone
the source media in one pass, creating a multiplexed
clone media.
- If you only clone 3 save sets, NW will most likely do
this sequentially and therefore will read through the
tape 3 times instead of one. So the cloning speed
reduces to 33%.
So there are a lot of things to consider if you investigate
cloning. And each scenario is different.
The most easiest way to prevent multiplexed data is to back
it up to a file device and clone the save sets from here. As
file devices never mix (interleave) save streams, such data
is ideal for cloning to tape (and for recovery, in general).
Hope this scheds some light.