On Thu, Mar 20, 2014 at 1:59 PM, Timothy J Massey <tmassey AT obscorp DOT com> 
wrote:
>
>
> Do you *REALLY* think this is some sort of binary on/off enhancement?  Have 
> 4GB?  No benefit.  5GB?  WOW Look at it go!!!   If you see zero improvement 
> with 8 TIMES AS MUCH RAM (when you're not swapping to begin with!), you 
> expect me to believe that double or triple that and all of a sudden a big 
> difference?  Come on now...

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...).  Swapping isn't the
issue, it is what read-ahead gets you for free if you don't throw it
away for lack of space.  I don't understand that 300Mb cache number
you mentioned earlier.   Linux normally uses all available memory for
disk buffering and discards it transparently if an application needs
it.  Also if you are short on memory and writing aggressively you may
force the system to flush the write cache before the optimal chance to
gather nearby locations.


> > > Of course, I have a minimum of 4 drives in a RAID array (6 minimum
> > for RAID-6), so I'm usually bottlenecked somewhere else anyway, such
> > as a single 1Gb link.  It's not hard to mange writing 70MB/s of data!
> >
> > Seriously?  You have rsync pegging a 1Gb link for long periods of time
> > anytime but the first run?  Do you have something creating a lot of
> > huge new files?  My times are mostly constrained by the read time on
> > the targets with a relatively small amount of data transfer.
>
> You have stated that you're limited by the TARGET.  So given that, who cares 
> if the disks are 60% busy instead of 30% busy while it waits on those 
> targets?  And why WOULDN'T I use something like RAID-5 or -6 and give myself 
> a LOT more usable space?

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.

> 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.

-- 
   Les Mikesell
      lesmikesell AT gmail DOT com

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