[5377] in linux-scsi channel archive
Re: Dumb question: Which is "better" SCSI or IDE disks?
daemon@ATHENA.MIT.EDU (Christopher Smith)
Thu Dec 10 00:36:10 1998
Date: Wed, 9 Dec 1998 07:13:15 -0500 (EST)
From: Christopher Smith <cbsmith@envise.com>
To: Gregory Maxwell <linker@z.ml.org>
cc: Leonard Zhang System Administrator ISD RVIB <leonardz@rvib2.rvib.org.au>,
linux-kernel@vger.rutgers.edu, linux-scsi@vger.rutgers.edu
In-Reply-To: <Pine.LNX.3.96.981209101625.14969B-100000@z.ml.org>
On Wed, 9 Dec 1998, Gregory Maxwell wrote:
> On Tue, 8 Dec 1998, Christopher Smith wrote:
> > On Tue, 8 Dec 1998, Leonard Zhang System Administrator ISD RVIB wrote:
> > > RAID-1 is disk mirror. DPT is cache with RAID. RAID will increase the
> > > disk performance.
> > Ugh.. FYI, RAID does not always increase disk performance, and in
> > particular RAID-1 is guarunteed not to increase disk performance by it's
> What? Join the real world but. Raid-1 doubles read throughput and the
> slowdown of writes is negligable.
A correctly behaving RAID-1 system needs to be able to account for the
fact that given disk will fail, so interleaving reads is not exactly a
good idea.
> > very nature (ok, it's conceivable that read performance could be
> > increased somewhat, but write performance would, if anything, be decreased
> > somewhat). Again, if the cache is actually playing a role in this
> > benchmark, then what you are really saying is that IDE is so fast that
> > it's 4x faster than DPT's cache memory. Think about that for a minute.
> Come off it, when you do disk benchmarks you use a dataset sufficently
> large so that the cache doesn't matter. Linux does caching just fine,
> DPT's cache is somewhat redundant.
His system wasn't Linux, it was SCO. If you looked at his data set size,
it was 9 MB and his cache was 32 MB. So yes, his benchmark was completely
inappropriate which is what everyone is trying to point out here.
> Almost all other raid levels perform worse (like raid 5 where the fastest
> write takes 2 reads 1 block XOR and 1 write).
You are talking about latency, not thoroughput. Read up on RAID, RAID-0 is
not the only version of RAID with performance advantages.
> The fastest raid array is a combo of raid-0(striping) and raid-1.
Why would this be faster than just RAID-0?
--Chris
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