[5378] in linux-scsi channel archive
Re: Dumb question: Which is "better" SCSI or IDE disks?
daemon@ATHENA.MIT.EDU (Gregory Maxwell)
Thu Dec 10 01:56:34 1998
Date: Wed, 9 Dec 1998 14:48:09 -0500 (EST)
From: Gregory Maxwell <linker@z.ml.org>
To: Christopher Smith <cbsmith@envise.com>
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.4.04.9812090706550.9855-100000@localhost>
On Wed, 9 Dec 1998, Christopher Smith wrote:
> 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.
No. A correctly operating raid understand that modern HDDs have ECC and
such and can report when the data is unreadable. There is no need to read
the data twice.
> > 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?
Take four drives. Here are raid-0 block numbers:
d1 d2 d3 d4
1 2 3 4
5 6 7 8
9 10 11 12
Now read blocks 1,5,9. Wow the performance is that of one drive.
Now raid-0+raid-1
d1 d2 d3 d4
1 1 2 2
3 3 4 4
5 5 6 6
7 7 8 8
9 9 10 10
Now read block 1,5,9.. Geez 1,5 are read in parallel!
Also comapir reading 1,5,2,6 All are read in parallel.
Raid 1 guanrees a doubling of read throughtput and slightly degrades
writeing.
Raid 0 gives you a chance of speed improvement (for 2 drives, 2x is 50%)
for both read and write.
Raid0+1 increases your chances of improving speed, and really doesn't hurt
write over a two disk raid 0.. The price is lost storage, the advantage is
speed and redundancy.
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