Greg Troxel writes:
>   Setting it up initially, I raided the 4 drives together with two
>   partitions on each of the components: a small one for RAID-1 to load
>   the kernel, and a large one for RAID-5.  Unfortunately, the RAID-5 had
>   horrible performance: 2-3MB/sec sometimes and never higher than about
>   12MB/sec.
> 
> I am not 100% clear on this, but I have the impression that RAID-5
> requires read-modify-write 

There is no 'requirement' for that.  You might end up having to do 
that, depending on configuration settings, but the reality is that 
you want to avoid r-m-w as much as possible.

> and that in the event of system crash or
> power loss you can get corruption, 

What sort of corruption would you like? :)  In general, the filesystem
corruption that you'd see is about the same as what you'd see with a 
single disk.  Where there is a chance for "more corruption than with 
a single disk" is if a component fails before parity has been 
completely verified.  In that case, if incorrect parity is used to 
reconstruct lost data, that lost data might be gone for good (and bad 
data returned).  Of course, you could get that same sort of bad data 
on any filesystem that overwrites files in-place, and the system 
happens to go down at an inopportune moment...

> and thus the good hardware
> controllers have a) battery backed RAM and b) code that won't crash.  (I
> don't mean to malign the raidframe code - but because it's in-kernel if
> the kernel crashes for any reason - not unheard of - then pending raid
> writes may not happen.)

Right.  Just like any other pending writes to anything won't happen.
 
> Because of this I've always just bought two big disks and done RAID-1.

RAID 1 has the same issue -- Say the machine dies at the point where
block n is written to component 0 but not to component 1.  If 
component 0 dies before block n gets synced between the two, then 
when you read block n from component 1, you're going to get the old 
data.

> Perhaps Greg Oster will chime in, and it would be a good addition to the
> Guide to discuss the wisdom of using RAID-5.
> http://www.netbsd.org/docs/guide/en/chap-rf.html

The reality is that RAID (hardware or software) is not a panacea, and 
you're just attempting to bump the odds in your favour.  And just like 
there is a non-zero chance of corruption, there is also a non-zero chance 
that all your disks won't die at once, or that a power surge doesn't 
take out your hardware raid configuration.

Personally, I like the odds that I'll be able to get a parity rewrite 
done in time, vs. the odds of having my only disk die. :)

Later...

Greg Oster