On Thu, Apr 29, 2010 at 03:15:15PM -0400, George Sinclair wrote:

> If you clone all the save sets on a given tape as:
> 
> nsrclone -b 'clone pool' -S -f file
> 
> where 'file' contains a list of all the ssids on the tape, and the
> save sets clone successfully to the target tape, then have you
> really validated anything?

You've read all the savesets in file, which does some verfications for
you.  Mainly that the tape is readable and that the NSR
headers/indications are intact.

> In other words, let's suppose instead that you cloned maybe half of
> the save sets using the same method. In this case, because the save
> sets are interleaved with others that may not be in the list, it's
> going to have to demultiplex those in order to read them so that it
> can then write them to the target clone tape, right?

Depending on what exactly you mean by "demultiplex", it would have to do
the same thing in the first one as well.  In both cases, it has to
identify the specific streams so that it can match against your SSID
list.  It's not doing more work in the second case.

> In this
> situation, I can see that NW is actually reading the requested data
> and writing it to the clone target volume.

> BUT, in the first scenario, wherein you specify all the ssids, NW is
> able to more efficiently clone them because it doesn't have to
> demultiplex anything, correct?

How does the second scenario demultiplex when the first one does not?
They're both making a single pass through the tape, and identifying
whether each part matches an SSID on your list.  If there's a fragment
on the tape that has no matching SSIDs in it, then it can skip that
fragment.  That would be the only difference.

> [...] BUT, in retrospect, I'm wondering if the former method
> is a better test or validation as it has to demultiplex since it's
> working on a subset? OR, is running on the whole thing good enough?

Working on a subset doesn't cause it to demultiplex any more than
working on a full set does.

-- 
Darren

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