[ILUG] Dell PERCs and Linux

[Paul Jakma]

On Sun, 18 Jun 2000, David Murphy wrote:

> I have only ever argued that 2-disk RAID1 is a better choice than
> 3-disk RAID5.
> 

and on which criteria were you arguing? speed wasn't it?

You said:

"RAID5 is relatively safe and cheap. It's not relatively fast."

> Nobody has yet posted 2-disk RAID1 v 3-disk RAID5 numbers on the same
> hardwre,

one Vs. the other, no. But we have at least seen the RAID5 benchmarks
with DAC1164's:

4 disk RAID5: 37MB/s block read, 8MB/s block write.[1]
3 disk RAID5: 34MB/s block read, 17MB/s block write.[1]

That clearly shows that RAID5 can be /very/ fast, contrary to what
you say.

> therefore there has been nothing but speculation on all
> sides, mine included. 

no. I have /seen/ the figures from linux software raid with my own
eyes: 

2 disk RAID1 wasn't much of an improvement over single disk, 
3 disk RAID5 was a vast improvement. (~ x 1.5).

> I have benchmarked nothing, and haven't
> suggested otherwise. I said that I did not know what linux did with
> reads from a RAID1 device. I don't know how you get from there to a
> suggestion that Linux software raid is deficient.

s/is defficient/perhaps suboptimal/

that was the implication anyway.

-- 
Paul Jakma	paul at clubi.ie
PGP5 key: http://www.clubi.ie/jakma/publickey.txt
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Fortune:
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[1] all arrays had write-through set. Write performance would be
better if they were write-back.