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External hard drive advice please



 
 
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  #1  
Old June 14th 18, 09:39 AM posted to alt.comp.os.windows-10,alt.windows7.general,alt.comp.os.windows-8
Chris
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Posts: 224
Default External hard drive advice please

Wolf K wrote:
On 2018-06-13 14:56, mechanic wrote:
On Tue, 12 Jun 2018 20:21:53 -0400, Wolf K wrote:

Probability of failure of all three at once will be 1/9th of
probability of failure of any one of them, which IMO is low
enough. :-)


Eh?


If I remember probability theory correctly, then if each device has the
same probability of failure, then the probability that two will fail at
the same time is (1/2)^2. If there are three, it will be (1/3)^2. And so
on. This is the reason that a RAID system is more reliable than any of
the drives in it.

If I've misremembered the probability math, kindly correct it (and save
me the work of checking it myself. :-) )


It's not quite right. The probability of three independent disks all
failing at the same is the cube of the individual probability: P(A)^3

If the individual probability is (not at all realistic) 0.1 (10%), then the
probability of all three is 0.001 (0.1%).

RAID is a bit complicated as the probability of a failure is highly
correlated. Firstly, the potential risk failure is if *any* of the disks
fail, which is the sum of the probabilities which for a 5 disk array is 5 x
0.1 = 0.5 (50%). Again not realistic.

Probability increases with age of the disk, so a 5 year old drive is more
likely to fail than a brand new one. This is where correlated failures
occur, particularly with RAID5, as when a disk fails the array needs to be
rebuilt putting a large strain on the existing (likely old) disks which can
cause another one of them to fail. The array is now dead and unrecoverable
which is one reason why RAID5 is not recommended.

In terms of raw disk failure probabilities RAID arrays are no more reliable
than separate disks, however, the redundancy and checksums allow for
seamless recovery from failures.

This is why a RAID is not a backup.




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  #2  
Old June 14th 18, 10:01 AM posted to alt.comp.os.windows-10,alt.windows7.general,alt.comp.os.windows-8
Paul[_32_]
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Posts: 7,573
Default External hard drive advice please

Chris wrote:
Wolf K wrote:
On 2018-06-13 14:56, mechanic wrote:
On Tue, 12 Jun 2018 20:21:53 -0400, Wolf K wrote:

Probability of failure of all three at once will be 1/9th of
probability of failure of any one of them, which IMO is low
enough. :-)
Eh?

If I remember probability theory correctly, then if each device has the
same probability of failure, then the probability that two will fail at
the same time is (1/2)^2. If there are three, it will be (1/3)^2. And so
on. This is the reason that a RAID system is more reliable than any of
the drives in it.

If I've misremembered the probability math, kindly correct it (and save
me the work of checking it myself. :-) )


It's not quite right. The probability of three independent disks all
failing at the same is the cube of the individual probability: P(A)^3


In addition to the P cubed thing, there's also a
slightly more refined analysis you can do.

It takes into account the MTTR (Mean Time To Repair).

The idea is, when one disk fails, you buy another, and
spend time transferring the results to the new disk.
The system has a window of exposure, while one disk
is out of commission. The longer the time to resolve
this, the more of a factor it could represent to a
complete loss of service. Repairing (replacing) a single disk
when it fails, raises the availability of the system.

The result then, is a state diagram or other kind of
analysis model, that takes all the states, their probabilities
into account.

(Has some illustrations about carrying out such an analysis)

http://www.engr.usask.ca/classes/EE/...es/notes11.pdf

The duration of the MTTR is important too, as it affects the
possibility of complete failure. In the systems we developed
at work, this value was set to 72 hours, representative
of humans "taking the long weekend off" and not realizing
something was broken.

In the real world, frequently duplicated systems have
sufficient redundancy for "normal" kinds of requirements.
Going triplicated, you have to watch for unexpected factors
having a higher probability of happening, than the
narrow set of things you're studying. For example, with
three disks, say the AC power goes off and you're
denied access to the data (for a short time). That could
represent an event of importance if you absolutely
needed access to the data at all times (say it was
a long list of family phone numbers or something).

I was hoping to find a worked case of a parallel triplicated
system, but no such luck.

Paul
  #3  
Old June 14th 18, 11:35 AM posted to alt.comp.os.windows-10,alt.windows7.general,alt.comp.os.windows-8
mechanic
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Posts: 882
Default External hard drive advice please

On Thu, 14 Jun 2018 08:39:54 -0000 (UTC), Chris wrote:

It's not quite right. The probability of three independent disks
all failing at the same is the cube of the individual
probability: P(A)^3


Yes, that's what I had in mind - although the assumption of
independence must be questionable, that's what did for Three Mile
Island, as the enquiry revealed (if I'm remembering correctly).
  #4  
Old June 14th 18, 02:11 PM posted to alt.comp.os.windows-10,alt.windows7.general,alt.comp.os.windows-8
nospam
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Posts: 2,187
Default External hard drive advice please

In article , Chris
wrote:

RAID is a bit complicated as the probability of a failure is highly
correlated. Firstly, the potential risk failure is if *any* of the disks
fail, which is the sum of the probabilities which for a 5 disk array is 5 x
0.1 = 0.5 (50%). Again not realistic.


it depends on the raid.

with raid 0, *any* failure loses the array. raid 0 is for speed, not
redundancy.

with raid 6, *two* drives can fail at the same time and no data is lost.

Probability increases with age of the disk, so a 5 year old drive is more
likely to fail than a brand new one. This is where correlated failures
occur, particularly with RAID5, as when a disk fails the array needs to be
rebuilt putting a large strain on the existing (likely old) disks which can
cause another one of them to fail. The array is now dead and unrecoverable
which is one reason why RAID5 is not recommended.


while that's true, the reason raid 5 is not recommended is due to the
chance of an unrecoverable bit error, which with the density of modern
hard drives, is statistically a near-guarantee to occur, and if that
happens during a rebuild, the array is lost.

In terms of raw disk failure probabilities RAID arrays are no more reliable
than separate disks, however, the redundancy and checksums allow for
seamless recovery from failures.


a raid is significantly more reliable for all sorts of reasons, other
than raid 0 which is used for speed, not reliability.

This is why a RAID is not a backup.


no, a raid is not a backup because it's a single device.

the advantage of a raid (other than raid 0) is uptime. if a drive
fails, the raid stays running and users can continue to access their
data.

a business cannot afford downtime while a new drive is obtained and
they restore from a backup. a home user probably can.
 




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