MTBF of a RAID-0 system (or dual cpu/memory where one unit CAN NOT continue without the other) will always be lower than a single drive unless the standard deviation (they never quote SD) of the MTBF is zero. i.e they all fail simultaneously at MTBF and none before - pretty unlikely I think.
Neither will the MTBF be halfed unless 1/2 the devices fail immediately and the other half last exactly 2x MTBF. The reality for MTBF of a RAID-0 will lie in between. MTBF is not really of great use for our puposes. Disk drive MTBF these days are quoted at about 250,000+ hours (>28 years continouous use)! I certainly have my doubts about these accelerated testing methods. But whatever happens to MTBF for multiple inter-dependant drives will be pretty irrelevant for the lifetime of our usage of the device. Cummalative failure rate is a much more useful figure for us and for a small number of fairly reliable inter-dependant devices this is nearly an additive figure - but not quite. Seagate reckon about 3.41% (flat-line model) will fail during the first 5 years of use (assuming you only use it for 2400 hours a year [6 1/2 hours a day]) : http://www.seagate.com/docs/pdf/newsinfo/disc/drive_reliability.pdf To calculate the failure rate for multiple inter-dependant devices you need to find the product of the survival rate and subtract it from 1. eg. Survival rate for 5 years is 100%-3.41% = 96.59% = 0.9659 So : 2 drives in raid-0 configuration running for 5 years 1-(0.9659*0.9659) = 0.06006975 = 6.0% - note this is not quite double. 3 drives in raid-0 configuration running for 5 years 1-(0.9659*0.965*0.9659) = 0.088737622625 = 8.9% - even further from triple Long before 5 years (AA excluded of course - has he got bored and gone to irritate other mailing lists?) you will probably want bigger and better storage . You can calculate figures for different expected usage yourself. Other manufacturers and probably other seagate product ranges will vary. Steve PS. Call me picky, but I notice that Seagate's actual warranty failure rate exceeds or equals their so-called "conservative" flat-line model. The author should seriously consider becoming a politician :-) ----- Original Message ----- From: "Austin Franklin" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, November 12, 2001 11:05 PM Subject: RE: filmscanners: Best solution for HD and images > > 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. > >