Re: [BackupPC-users] Setting up a new BackupPC server

Jim Leonard wrote:
>
> You make it sounds like RAID-5 is incapable of saturating drive 
> bandwidth.  I haven't seen this on any modern (2-yr-old or newer) 
> machine with more than one CPU.  And he's using hardware RAID so the 
> point is moot anyway.

With a large number of drives raid5 should be OK.  With a small number, 
the need to read the last block and recompute parity before a write ties 
up most of the heads. Benchmarks doing large writes probably don't show 
this effect.  But, backuppc probably spends about as much time updating 
the link count and ctimes in inodes as it does writing file data.
> 
> I think you're overestimating the performance penalty of latency.  This 
> is worked around in most systems with caching (including a large amount 
> of RAM on hardware controller cards), and only for writes.

Yes, but there is only so much you can do for the pre-read that you need 
ahead of a partial block write.

>> raid6 shines with 10 or 12 spindles.  I say raid 6 because I wouldnt 
>> risk a large array to a single drive fault and a hotspare has a rebuild 
>> window that makes me nervous.  raid5 likes odd numbers of drives 
>> active(not including hotspare) and raid6 like even numbers.  I cant give 
>> a scientific explanation and can only explain it is a phenomenon.
> 
> It is explainable if you know how many columns are in your RAID setup. 
> Number of drives isn't important; an integral multiple of drives 
> compared to the number of columns is.

The number of drives relates to how much chance you have to do something 
else while at least 2 or 3 are tied up in the read/recompute/write cycle.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com

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