Re: 4 X 500 gb drives - best software raid config for a backup server?

[Chan Chung Hang Christopher <christopher.chan@xxxxxxxxxxxxxxx>]

Ian Forde wrote:
> On Fri, 2009-02-20 at 22:52 +0800, Chan Chung Hang Christopher wrote:
>   
>> Bollocks. The only area in which hardware raid has a significant 
>> performance advantage over software raid is raid5/6 given sufficient 
>> cache memory and processing power.
>>     
>
> I'd have to say no on the processing power for RAID 5.  Moore's law has
> grown CPU capabilities over the last 15 or so years.  HW RAID
> controllers haven't gotten that much faster because they haven't needed
> to.  It's faster to do it in software, though it's preferable to offload
> it to HW RAID so that any apps aren't affected directly.
>   
You will have to prove that. I have previously posted posts with links 
to benchmarks that show that hardware raid with sufficient processing 
power beat the pants of software raid when it comes to raid5/6 
implementations. Hardware raid cards no longer come with crappy i960 cpus.
> I would agree on that cache memory is an advantage, especially when
> considering battery-backed cache memory.
>   
There is more to it. That cache memory also cuts down on bus traffic but 
the real kicker is that there is no bus contention between the board's 
cpu and disk data whereas software raid needs to read of the disks for 
its calculations and therefore suffers latencies that hardware raid 
boards (which have direct connections to disks) do not. Of course, if 
the cache size is insufficient, then the hardware raid board will not 
perform much better if not worse than software raid.
> But those aren't the only significant areas.  HW RAID allows for
> hot-swap and pain-free (meaning zero commands needed) disk replacement.
>   

Hmm...really? I guess it depends on the board. (okay, okay, thinking of 
antique 3ware 750x series may not be fair)
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