On Wed, May 13, 2009 at 03:22:24PM -0400, George Sinclair wrote:
> A Darren Dunham wrote:
> >On Wed, May 13, 2009 at 12:51:03PM -0400, George Sinclair wrote:
> >>When NetWorker is backing up a *single* save set, does it send each file 
> >>over one at a time or does it wrap two or more files together into that 
> >>one stream?
> >
> >Hmm.  I'm not sure what the distinction is here.
> 
> Perhaps a better way to phrase my question is that if the content of 
> more than one file was ever being sent to the backup server at the same 
> time then is it the case that those would have to be from different save 
> sets (multiplexing)? Or is it possible that NW could somehow chose to 
> read the content of multiple files simultaneously (when only a single 
> save set is being backed up) and somehow wrap them together into one stream?

Got it.  I think I understood that better.

> If I infer your response correctly, the answer is that only one file is 
> sent at a time when using a single save set. Tar doesn't have the 
> ability, however, to multiplex. I mean, you can only tar a single stream 
> to a tape at a time, but NW can interleave multiple streams (save sets), 
> but only one file at a time is being sent in each stream; any given 
> stream does not consist of multiple files wrapped together??

Correct.  I think of this as two separate processes: a filesystem
serialization, and a tape communication.

When thinking of the filesystem serialization (which happens on the
client), 'save' and 'tar' are very similar.  Neither one does any real
parallelization, they're just walking the filesystem one file at a
time.  If I want to do two things at once on the client, I have to run
multiple copies of 'save'/'tar'.

Networker multiplexing and tape communication happens on the storage
node.  Multiple streams are pushed to the tape simultaneously.  But
that's not something that any individual 'save' sees.

-- 
Darren

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