[Gllug] Re: RAID on laptop or xfs?

[Bruce Richardson]

On Sat, Apr 02, 2005 at 12:00:29PM +0100, Martin wrote:
> Bruce Richardson wrote:
> 
> >Oh, certainly.  But I'd rather have either of them running on SCSI than
> >IDE.  The point in this thread has been the unreliability of IDE, not
> >journalling.
> > 
> >
> 
> Which, to my knowledge, has been mythical for several years, at least 7 
> or 8.

By second hand knowledge of one manufacturer, you mean.

> 
> I used to know a chap who worked for Western Digital. He told me that 
> only difference between IDE and SCSI drives manufactured by WD was 
> interface electronics and warranty hence the price difference.

And which bit did you think the writeback cache was on?  Which bit do
you think contains the logic and hardware that makes the SCSI interface
so much more intelligent, resilient and capable of higher throughput and
parallelism?  You seem to be confusing the quality of the moving parts
with the quality of the controller.

I'm sorry, but what you say just doesn't correlate with my experience of
building large storage arrays and the anecdotal and researched evidence
of many others.  Most IDE and SATA drives ship with the writeback cache
enabled just to try and match the performance of SCSI drives and even
then they aren't as good.  Try initialising a 1T RAID array on SATA and
then on SCSI and see the difference.  I had occasion to do that just
this week and the gap in performance is noticeable.

What's worse is that disabling the writeback cache on IDE not only
seriously degrades performance but in many cases also invalidates the
warranty.  Several manufacturers explicitly warn that recovery is not
supported for IDE or SATA drives that have the cache disabled.  In
contrast, while a SCSI drive may have a write-back cache, they are
usually disabled by default (and don't actually offer a significant
gain over normal SCSI performance, in any case).

I could take your seven year old conversation as conclusive proof, or my
own experience and the careful research of quite a few people whose
results are easily available on the Net.

http://sr5tech.com/write_back_cache_experiments.htm

-- 
Bruce

Hierophant: someone who remembers, when you are on the way down,
everything you did to them on the way up.
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