RE: filmscanners: Best solution for HD and images

["Austin Franklin" <darkroom@ix.netcom.com>]

> Seems like you have done everything and also know everything.

Not everything, but having been an engineer for 25 years, I have done many
projects including digital imaging systems, and SCSI systems...  What I do
know, I know, and what I don't know, I know I don't know.  I don't just make
things up.

> I don't
> know how your company  (or you) determined MTBF of a RAID0 system but
> most companies as Compaq, IBM, Sun, Adaptec, etc. say that MTBF will
> decrease.

There is only one article I have seen that says this, and I have had
discussions with the authors about this.  Do you have any reference to
articles/spec sheets that make this claim?

Interestingly enough, MTBF does not derate for adding a second CPU or for
adding more memory to the system...

> Exactly because of the reduced MTBF of a system with multiple
> HDs Berkeley has suggested the RAID system.

Is this "study" published anywhere?  If so, I'd like to see it.

> The RAID system is supposed
> to relax the impact of the reduced MTBF. That doesn't mean the MTBF
> becomes higher when a RAID system is deployed but it just makes it more
> likely that the failure can be repaired.

Failure recovery is entirely different from MTBF.

> I see though where your (company's) calculation might come from.

The company was Digital, BTW.  We had an entire department devoted to MTBF
testing...and specifically to storage MTBF assessment.

> You
> can determine MTBF for a certain device by testing for example 10000
> drives for 1000 hours and then divide the total of 10000*1000 hours by
> the number of failures.

That's not really how you determine MTBF.  MTBF is an average.  You are
right, you need a large sample to test though.

> Nevertheless, this calculation doesn't apply to RAID as a RAID system
> has to be considered as a single identity.

Exactly, and that is why you don't get any decrease in MTBF by adding
drives.  It's really simple.

> So you cannot claim that
> because you have 10 HDs your RAID system is working 10*1=10 hours in
> each single hour. Your RAID system is ONE identity and therefore is
> working only 1 hours each hour it is up. Therefore the MTBF decreases.

Why does the MTBF decrease?  You have a magical "therefore" that doesn't
follow.

If you tested 1000 drives by themselves, and you got an MTBF of 1,000,000
hours, let's say...take those 1000 drives, and make 500 RAID 0 systems, and
your MTBF will NOT decrease notably, if at all, from drive failure.  It may
from other factors like power supply or thermal, but not from drive failure.