Re: Four Drive RAID-5 on RAIDFrame Considered Harmful...

On Oct 10, 2007, at 4:48 AM, Greg Troxel wrote:

>   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

Yep, unless you can do "full-stripe" writes, which is why you want  
the filesystem block size to match the stripe size...

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

Well, I'm not that familiar with the 'Raid-5 hole', but Wikipedia  
seems to indicate that the parity can get wrong on a crash, and  
unless you detect that before a failure, you'll lose data.  But since  
RAIDFrame always regenerates the parity when it's not shutdown  
cleanly, that shouldn't (?) be a problem...I think :-)

> Because of this I've always just bought two big disks and done RAID-1.
>
> 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

I thought about cross-posting to tech-kern, since it seems like that  
gets a fair amount of RAIDFrame traffic, but my initial concern was  
primarily with the performance I was seeing, and I know cross-posting  
is frowned on.

Thanks,

Robert

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