Subject: Re: ahc and raidframe questions
To: Greg Lehey <email@example.com>
From: Curt Sampson <firstname.lastname@example.org>
Date: 06/26/1999 21:56:18
On Sun, 27 Jun 1999, Greg Lehey wrote:
> A lot of this depends on what you're trying to do. RAID-3 is good for
> things like video which require high transfer rates with sequential
> access, but it won't help much for massively concurrent applications
> such as web and ftp servers, since positioning takes too long.
> I still don't have an opinion about the difference in performance
> between "software" RAID and "hardware" RAID. They're both software
> RAID, of course: the real difference is just where the software gets
Perhaps I didn't make myself clear. The whole point of an external
hardware RAID box is that it's got specialised hardware and software
for dealing with a RAID array that is hard to duplicate in software
RAID on a host. Parity calcuations are not a big deal, I know, but
here are some other advantages (assuming the hardware RAID is done
right, of course):
* Use of a large, battery-backed cache to group your writes
lets you use RAID-3 instead of RAID-5 for faster write
throughput while helping to avoid the small write penalty
inherent with RAID-3 (and making sure that your large-write
applications perform better than they ever could with RAID-5).
* You use spindle sync means that to reduce your average time
to access data after a seek from something close to a revolution
to a half-revolution.
* You have two controllers in the box, and two controllers on
the host, so that if a controller fails on either the box or
the host, or a cable fails, things keep running.
> I'd be interested in collecting them. Again, I'd
> plug my rawio program (ftp://ftp.lemis.com/pub/rawio.tar.gz), which
> bypasses buffer cache and thus gives more accurate results for the
> underlying storage equipment.
You should add this to pkgsrc. I currently use bonnie because I
prefer to test file-system I/O rather than raw I/O speeds, but now
that we're about to get a unified buffer cache, and I have a 512
MB sparc, I don't think I'm going to have the patience to do bonnie
with a big enough file to know throughput. :-)
Curt Sampson <email@example.com> 917 532 4208 De gustibus, aut bene aut nihil.
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