Subject: Re: Four Drive RAID-5 on RAIDFrame Considered Harmful...
To: Greg Troxel <firstname.lastname@example.org>
From: Robert P. Thille <email@example.com>
Date: 10/10/2007 08:42:45
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
> 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.
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.
Robert Thille 7575 Meadowlark Dr.; Sebastopol, CA 95472
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