[Bioclusters] Urgent advice on RAID design requested

[Angulo, David]

OK, but the MTBF quoted by manufacturers is the MTBF of the entire unit.  All components together.  Why talk about all this theoretical items when you can compare actual MTBF stats?


-----Original Message-----
From: bioclusters-bounces+dangulo=cti.depaul.edu at bioinformatics.org on behalf of Joe Landman
Sent: Thu 1/18/2007 10:39 AM
To: HPC in Bioinformatics
Subject: Re: [Bioclusters] Urgent advice on RAID design requested
 
Angulo, David wrote:
> you say your point stands.  I say it does not.  Please compare the actual MTBF figures.

Hi David:

   The MTBF of the system is related to the MTBFs of all of the 
components.  If the MTBFs of the disks are so large that the power 
supplies and RAID card or other components MTBFs are lower, the latter 
will dominate the MTBF.

Take 5 of these units.  I am seeing MTBFs quoted as 10000 to 100000 
hours for the enclosures.  For laughs, lets take 20000 hours.  There are 
8760 hours per year.  5 of these units would consume 43800 operational 
hours per year.  For a 20000 hour MTBF for these units you should expect 
a rough failure rate of about 1 enclosure failing every 5 months or so.

You can ameliorate some of this by building mirror images of these 
units.  Then you need to worry about the other MTBFs which might not be 
so well documented.

Lets for the moment stipulate that the disks themselves are infinitely 
reliable (they are not, but that is not the point) with zero failure 
rate.  The other elements of the equation are not as reliable and will 
fail.  Things like power supplies have MTBFs ranging from 10000 through 
100000 hours.  What are the MTBFs of the cables, the USB2 ports, etc? 
Is there data on this?

The issue at the end of the day is that what you dont expect is usually 
what bites your data.  Limiting the maximum damage it can do (N+1 
supplies, multiple redundant nets, ...) before you can service it is one 
of your few options.

Joe

-- 

Joseph Landman, Ph.D
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