Les Mikesell <lesmikes...@gmail.com> wrote on 03/20/2014 03:48:51 PM:
> It's all about statistics and the odds of having to move the disk head
> to get a directory entry or inode vs already having it in cache for
> instant access (and sometimes even some data...).
In theory, theory and practice are the same. In practice, they aren't.
(Or, another way: theoretical analysis doesn't always make a difference
in the real world.)
You keep telling me the theory of why more RAM would help. I keep telling
you the FACT that it doesn't. Repeating your theory doesn't change the
FACT that it doesn't help.
(Why don't you try it sometime? Either use a kernel parameter or remove
some RAM from one of your magical 20GB boxes. You stated that even 4GB
was not enough to see the effect you're describing, so do that. I *HAVE*
done this test. Several times. You only have to do it for a single full
backup of a single large target: one night and you'll have a FACT to
analyze, not a theory.)
> I care because _some_ of my targets are relatively fast - and have new
> big files daily. These will routinely show a 40+ MB speed in the
> backuppc host summary, although I'm not exactly sure what that is
> measuring. 10 or 12 is more common for run-of-the-mill targets, even
> less if the files are mostly small or the server is older and slower.
If you don't know what it's measuring, how can you use it for anything.
> > And my point was not that I *am* limited by my 1GbE link (I'm not
> usually with BackupPC but I *am* with other uses of my backup
> servers in general),
>
> It seemed like you were advising to expect backuppc to be limited by
> bandwidth - which doesn't match my experience at all once you get the
> initial copy.
Nope. Merely (and somewhat awkwardly) making the point that it is
*meaningless* to make your disk performance faster (as in huge RAM caches
and avoiding parity RAID) if it's already several times faster than some
other bottleneck.
The idea encouraging someone to not using RAID-5 or -6 on a BackupPC box
is just about the biggest waste of money I can possibly imagine for such a
machine. Well, maybe encouraging someone to use 15,000 RPM SAS drives not
in a parity array might be worse... :). The cost of hard drives *always*
exceeds 50% of my array-based (as in not single drive) BackupPC servers.
Why would I want to make that storage even *more* expensive if it buys me
no measurable difference in performance?
And make no mistake: I have not personally seen an application where lack
of RAM (where that RAM is measured in Gigabytes) or disk performance (with
4 or more drives, let's say) has led to a bottleneck on my systems. Now,
I've not tried backing up more than low double digits of targets to a
single server, and I've not done much more than 10TB of pool space: maybe
if you scale significantly beyond that, maybe. But even then I'd need to
see it, not just theorize it...
And I'm officially done! :) (Unless there are hard facts presented.)
Tim Massey
Out of the Box Solutions, Inc.
Creative IT Solutions Made Simple!
http://www.OutOfTheBoxSolutions.com
tmas...@obscorp.com
22108 Harper Ave.
St. Clair Shores, MI 48080
Office: (800)750-4OBS (4627)
Cell: (586)945-8796
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