Tyler J. Wagner wrote at about 17:24:29 +0000 on Tuesday, March 22, 2011:
 > On Tue, 2011-03-22 at 13:09 -0400, Jeffrey J. Kosowsky wrote:
 > > I think it would still be helpful to have a hook that would allow some
 > > type of query of bandwidth available. Perhaps most users would never
 > > use the hook but having the option to call an external routine and
 > > return a bandwidth availability metric would be nice...
 > 
 > It's good to discuss features, but I don't think it is productive to
 > discuss features that don't exist on the underlying platform, or are
 > outside the scope of BackupPC.

Much of BackupPC, by your definition, would then not qualify,
including all the transport methods, ping, nmblookup, etc.


 > 
 > How do you get a bandwidth availability metric? Short of trying to push
 > data back and forth, you can't.

That's the whole point. BackupPC wouldn't need to do anything. It just
would have a hook that would run a shell command or perl code (just
like all the existing hooks) and whose return value would then be
interpreted as a bandwidth availability. You could set it to 'undef',
call some 3rd party program, use your perl code, or return a fixed
number. Whatever you want. That's the beauty of the flexibility of a
program like BackupPC -- consistent with the *nix philosophy, it has
the ability to leverage other code rather than re-inventing the wheel
and integrating it into the core code.


 > Once you have that, how do you limit it? Again, this is dependent on an
 > existing bandwidth control system like TC, or the individual backup
 > tools supporting it (like rsync --bwlimit). BackupPC is just a (very
 > awesome) front-end to the backup tools that do the copying. Not all of
 > those tools have an in-built bandwidth limit system.

The point is to allow modular extensibility of BackupPC to include
important user requirements. If this is something important to many
users, then I'm sure it wouldn't be hard to:
- Determine a robust and well-defined framework for implementing the
  desired capabilities.
- Provide one or more examples for specific implementations

This is how things like transport methods are done. There is a general
framework that can be extended to almost any 'copy/archive' method and
then specific examples like rsyncd, tar, smb etc. are implemented.

 > I'm just trying to limit suggestions to what the developer would
 > actually listen to, or could reasonably implement.

I'm just trying to understand what are potential user interests and
requirements and to allow for a discussion of pros/cons and
feasibility before shutting down the discussion by 'limiting
suggestions' and determining what one 'could reasonably implement'
before we even know what users want and how hard it would be.

By the way, hooks are about the easiest possible thing to implement
e.g., If defined hook, execute hook. If return value = 0, don't run backup
          or run it niced  or some other desired behavior etc. 

Pretty darn simple.

Personally, I have no major need for bandwidth control. But given that
bandwidth is often the rate limiting step and that non-optimal
bandwidth utilization can cause backups to bog down, it seems very
reasonable to discuss whether and how bandwidth capacity and
utiliziation can be used to optimize BakcupPC performancd

 > 
 > Regards,
 > Tyler
 > 
 > -- 
 > "If one would give me six lines written by the hand of the most honest
 > man, I would find something in them to have him hanged."
 >   -- Armand Jean du Plessis, Cardinal Richelieu
 > 
 > 
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