tmassey AT obscorp DOT com wrote:
>
> I have a philosophical problem with compression and backups (my goal is to 
> have my backups as simple as possible), so I've never used compression. So 
> take what I'm going to say with a grain of salt.  But:

You can oversimplify things though...

> In short, the only thing that requires RAM is the file list, whether 
> compression is used or not.  Like I said, I've done servers with ~250GB of 
> data and half a million files with 512MB of RAM, of which more than 100MB 
> was still being used as cache, and exactly zero bytes of swap. 
> 
> So unless you're going to have servers with literally millions of files 
> (e.g. mail or Usenet servers that store each message as a separate file), 
> 512MB of RAM is plenty.  It's not like disk cache is going to help you on 
> your backup server:  it's merely streaming data to disk.  100MB of disk 
> cache is *plenty* to hold the metadata it'll need...

That's just for the internal rsync comparison.  The real killer, time 
wise, is disk head motion which is thousands of times slower than any 
other computer operation.  Adding RAM that will automatically be used 
for disk cache will eliminate many disk seek operations that you'd 
otherwise be waiting for, and compression will reduce the total of 
necessary disk accesses and enable more to fit in the cache.  And 
compression may be particularly important if you are trying to make your 
  archive fit on a single disk for ease in rotating offsite copies.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com


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