10-20-2008, 10:23 PM
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| Junior Member | | Join Date: Sep 2009
Posts: 0
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 Re: SCSI vs SATA hard disks
Whoever wrote:
> On Mon, 20 Oct 2008, Barry Keeney wrote:
>> criten wrote:
>>> Hactar wrote:
>>>> In article <87d4ixbd77.fsf@teufel.hartford-hwp.com>,
>>>> Haines Brown wrote:
>>>>> ebenZEROONE@verizon.net (Hactar) writes:
>>>>>
>>>>>> In article <87hc89bmfm.fsf@teufel.hartford-hwp.com>,
>>>>>> Haines Brown wroteready worko), there
>>>>>>> Would a move to a SATA 3.0 Gb/s drive such as the Seagate Barracuda mean
>>>>>>> that I will henceforth have to accept drive unreliability?
>>>>>> _All_ drives are unreliable to some degree. The ultimate in
>>>>>> computer-readable reliability is probably Tyvek punched tape.
>>>>> Yes, but my subjective impression is that there is a very wide
>>>>> difference in reliability. Of the dozen SCSI drives I've used over the
>>>>> years, only one failed on me; reading on line discussions and reviews,
>>>>> it seems that SATA drives fail regularly.
>>>>>
>>>>> I guess my question comes down to, why should one bother these days with
>>>>> the added expense of SCSI hard disks?
>>>>
>>>> It is my impression (which may be false and/or out of date) that the
>>>> instances of drive hardware that are matched with SCSI controllers are the
>>>> more reliable (longer-lasting) ones.
>>
>>> This is a common misconception. The interface is irrelevant,
>>
>> Well it makes a difference, just hard to say if the interface
>> improves the life of the drive.
>>
>>> the 'mean
>>> time failure rate' is most certainly relevant. Essentially you pay more
>>> for a disk with a longer mean time failure rate, meaning its less likely
>>> to fail.
>>
>> HAHAHAHHA!! Thanks, I needed a good laugh.....
>>
>> Sorry, Not putting you down, Just the numbers they toss out.
>>
>> I used to work for a company what wrote software for figuring
>> out the MTBF (Mean Time Between Failures) and spent a lot of
>> time working with reliability engineers. MTBF is a bit of a
>> guess at best.
> It's a pity you did not learn what MTBF actually refers to.
It's the "average time between failures of a system" that's what
it means.
I wasn't saying *HOW* it should be used in the big picture.
>>
>> Okay, Lets compare two seagate drives:
>>
>> Barracuda 7200.10 SATA 3.0Gb/s 500-GB Hard Drive (ST3500630AS)
>>
>> MTBF 700,000 hours (79 years!)
>>
>>
>> Barracuda ES.2 SAS 3.0-Gb/s 500-GB Hard Drive (ST3500620SS)
>>
>> MTBF 1,200,000 hours (136 years!)
>>
>> From the numbers about you might think the SAS drive is going to
>> last twice as long. I don't think either of these drives are going
>> to last 50+ years unused in storage let alone in a running system!
>> You might see 5-7 years of 24/7 running, at best, before they're
>> going to start to drop like flies.
>>
> That's not what MTBF is intended to measure. You are claiming that MTBF
> should equal lifetime and it does not.
No, That's not my claim, I know it's not. When you only see the MTBF
number it's easy to jump to the idea about how long something might last.
I'm claiming the value of the "MTBF" is just about useless. There are
much better ways for life cycle analysis/modelling. MTBF is great to
toss out but has no real value by itself out of any context.
Without knowing how the value for MTBF was calculated you can't know
it's usefulness. Was it from a steady failure model like the Mil standards
or something else? Whats the data behind the MTBF? How did they
get this number? Did they do any real run testing or just run the
numbers (ideal temp/operating conditions) that gets the best MTBF number?
"Hmmm if we run the drive at a temp of -5C, the calculations say the MTBF
is 1,200,000 hours. That's within the listed operating range."
I'm not claiming Seagate or any other drive company is lying, cheating or
trying to mislead people, they are just putting out the info they have
that puts their products in the best light, idea enviroment/best possible
results. You might find a paper on how they do their testing but it'll take
some digging to figure out how they got their MTBF or MTTF numbers for
a drive.
> Essentially, MTBF measures the likelihood of a random failure, NOT an
> end-of-life failure. Arguably, MTBF is only useful to people who run large
> datacenters with many disks -- they can use MTBF to estimate the failure
> rate of their drives.
No, MTBF is the *AVERAGE* time between failures. That's why I hate
seeing it used in marketing and specs sheets. It's not the real average, not
even close (for hard drives anyway). It's not real data from years of
running the drives, they don't have the time to run the drives for years
before sending them to market to get the real numbers. It's just, at best,
educated guessing using known data about the parts.
If you take a 1000 new drives and run them until each fail the average
you get won't be anything like 1,200,000 hours, even if you toss out
numbers first 100 failures and only use the 900 longest lasting drives
data.
MTBF can be useful during the early design of new devices/electronics.
If I get a value of MTBF of 1000 hours and I need atleast 2000 hours I
need to rework the design or use other methods to figure out why it's
low and fix the design.
MTBF isn't useful by itself. The Annual Failure Rate(AFR) might be more
useful, depending how they figured that out but no details on this either.
(AFR for the ST3500630AS is 0.34%, ST3500620SS is 0.73% )
How do I decide on a drive vendor?
I use warranties and how the company deals with warranty
repairs/replacement for drives as a guild. Not going to be
the only thing I look at but it has been useful to me.
A Short warranty - 3 years or less
Paying to upgrade the warranty to 4 or 5 years.
Limits for warranty replacement (only one warranty replacement, etc)
Having to pay shipping costs
These are possible problems and the drive might not be as good as
others or it's going costs more over the long run.
Is the warranty for their drives in their external cases the
same as internal drives?
If the maker can't build a case/drive combo that they will stand
behind as long as an internal drive, maybe I should look elsewhere.
--
Barry Keeney
Chaos Consulting
email barrykchaoscon.com
"Rap is Square Dancing gone terribly, terribly Wrong...."
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