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Re: choice of external USB drive for mount at boot
On Mon, 15 Jun 2015 14:41:03 +0100, Ahem A Rivet's Shot wrote:

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indeed & if they were all originally installed at the same time the  
chances of that happening are significant (I have seen it on more than 1  
occasion)




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Re: choice of external USB drive for mount at boot
On 15/06/15 15:23, alister wrote:
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Only if they are in the same chassis and got the same voltage spike ;-)

I have NEVER had that happen to me. And we are talking hundreds of machines

=

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Re: choice of external USB drive for mount at boot
On 15/06/15 14:41, Ahem A Rivet's Shot wrote:
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how likely is that?


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Re: choice of external USB drive for mount at boot
On Mon, 15 Jun 2015 18:56:21 +0100

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The rebuild can stress all the drives more than is sensible at their
age.


Re: choice of external USB drive for mount at boot
On Mon, 15 Jun 2015 20:31:38 +0100, Rob Morley wrote:

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...but there's a simple solution to that - don't use disks from the same  
batch, model, or make as your prime disk for your backup disk. Similarly,  
if you use more than  one backup generation, make the backup generations  
are also different, i.e. do exactly as is recommended for RAID mirroring.


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Re: choice of external USB drive for mount at boot
On Mon, 15 Jun 2015 18:56:21 +0100

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    If you have the replacement drive to hand, start the rebuild
immediately and are not using drives from the same batch in the array, and
the dead drive is an early failure then probably not very likely. However if
you have to wait two weeks for an RMA replacement to arrive (which has
happened to me twice) the odds go up sharply, if you built the array using
drives from the same batch the odds go up sharply and if all the drives are
getting near their expected life then the extra stress of running the
rebuild can kill one. Much depends on how many drives you have in the FEC
list too - if you have RAID5 with 2+1 then the odds are rather better than
if you have RAID5 with 50+1 - both arrays have single drive redundancy but
one requires 2 out of 3 drives to fail before you lose data the other 2 out
of 51.

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Re: choice of external USB drive for mount at boot
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Of course only a real fool would build a 50+1 RAID5 array, especially when
not having one or more hot spares.

However, in practice even with notoriously bad drives in servers at work,
it has never happened that a second drive in the array (we usually use
RAID1, sometimes RAID5 but only with max 5 drives) fails within months
of the first one failing under use.  And of course we get a replacement
drive within 4 hours of the support call.

It could happen, and it would be really best when different makes of
drives are used, but even manufacturers that source their drives from
different manufacturers usually don't put different ones in any particular
server they deliver.  So you would have to arrange for that yourself.

In fact I worry more when the power fails.  The chance that a drive fails
is significantly more when you cycle the power (and it won't come back
up).  That could happen to two drives in the array.  Never happened to us.

Re: choice of external USB drive for mount at boot
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What you see there is that the typical lifetime of a disk is 3-5 years,
and then most of them fail.  Under heavy use, for cheap disks, it is
shorter.  Good disks that are specified for continuous use still achieve
3+ years.

Your disks probably still have an MTBF of 20 years, but that only
expresses the number of failures within some specfied time.  I.e.
it tells you how many disks will fail in the second year.  Not how
long they will work on average.

Re: choice of external USB drive for mount at boot
On 15/06/15 13:08, Rob wrote:
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You are certifiably insane.

The MTTF of my disks is ~4 years. End of story., On the data I have  
given you.


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Re: choice of external USB drive for mount at boot
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I explained that here.

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You may have found a formula that is sometimes used to quickly arrive at
a MTxF, but it would be applicable to both MTTF and MTBF.
The above cannot be correct because the resulting dimension (unit) is
not time. So there must be another factor (length of each operation).

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No, it is important to note that although it is a "mean time", it is
not to be understood as an elapsed time before failure of the device.
It is "mean" because it expresses the failure rate for a large number
of devices, not because it averages the elapsed time before a unit fails.

Re: choice of external USB drive for mount at boot
On 15/06/15 13:04, Rob wrote:

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I may be goimng senile, but I cannot actually see any difrence.

We nave n devices  they fail after X(n) months,. The average of X(0--n)  
is the MTTF, it is the average life of the units.


What the 'failure rate for a large number of devices' is, if not that,  
is beyond me..

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Re: choice of external USB drive for mount at boot
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NO.
We have n devices and of those a certain percentage fails after 1 year.
THAT is what the MTTF expresses.  Not how long it takes for the average
unit to fail.

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The difference is the measurement period.
You are measuring how long it takes for a unit to fail, what the MTxF
people measure is what percentage of the units fail within some fixed
interval.  That is fundamentally different, especially when the interval
is short relative to the outcome of the calculation.

Re: choice of external USB drive for mount at boot
On 15/06/15 13:15, Rob wrote:
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No, it isn't. End of.

  Not how long it takes for the average
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Re: choice of external USB drive for mount at boot

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Yes it is.  Rob is right; you are wrong.

You appear to be unable to grasp the difference between MTTF and
lifetime.  There is no direct relationship between them.  The
only connection is that the MTTF applies during the lifetime.
The lifetime is a rating, determined by the manufacturer.

I know that the point of failure of a disc drive is the end of
the life of that particular specimen, but it's not the same
thing as the manufacturer's rated lifetime.

To be practical: if you have a large enough set of drives (which
means large enough to be a statistically representative sample)
and you run them, within their ratings, until the end of the
rated life as specified by the manufacturer, you would expect
that most of them are still operating correctly.  In the example
I quoted earlier in this thread (5 year lifetime, 1 million
hours MTTF), you would expect over 95% of them to be still good
after 5 years.

Dave

Re: choice of external USB drive for mount at boot
On 15/06/15 20:47, Dave Higton wrote:
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What you think MTTF is, is not what MTTF actually is,.

Go back and read up.


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Re: choice of external USB drive for mount at boot

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I have made exactly one post in this thread in which I asked a
straightforward question that you did not reply to. Let me repeat it.
What is the difference between the MTTF of a disk drive and the MTTF
of an elephant? Any answer, whether it is the formula used for the
calculation or why I should, or should not, use the number in choosing
a pet or a disk drive would be appreciated.
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I believe that if I base my decision on the MTTF of my pet I would be
better off with a pet elephant than with a pet fruit fly. I know that
a fruit fly might outlive the actual elephant I chose but,
nevertheless, I believe my choice is rational. Am I mislead? Why is a
similar choice based on the MTTF of disk drives not equally rational?

Re: choice of external USB drive for mount at boot

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He is making no such assumption.

The M in MTTF is for Mean, i.e. the average.

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But that isn't how MTTF is calculated.

Dave

Re: choice of external USB drive for mount at boot
On 15/06/15 20:52, Dave Higton wrote:
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It isn't calculated the way you think it is, else it would be a useless  
metric.



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Re: choice of external USB drive for mount at boot

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Mean Time Between Failures implies a repairable system, i.e. one that
may fail, be repaired, fail again, etc.  Mean Time To Failure is the
term to use on a system that will not be repaired after it fails.
Disc drives are normally not often repaired, so MTTF is more
appropriate.

But they mean the same thing really.

Dave

Re: choice of external USB drive for mount at boot
On 6/14/2015 9:20 AM, Rob wrote:
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I think this is not true.  The probability of a drive failing is a  
function of time and is typically not constant over the lifetime of the  
product.


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I think this may be key in what you are sayiing.  How then do they  
calculate the MTBF?

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