On May 1, 2010, at 1:56 PM, Bob Friesenhahn wrote:
> On Fri, 30 Apr 2010, Freddie Cash wrote:
>> Without a periodic scrub that touches every single bit of data in the pool,
>> how can you be sure
>> that 10-year files that haven't been opened in 5 years are still intact?
> You don't. But it seems that having two or three extra copies of the data on
> different disks should instill considerable confidence. With sufficient
> redundancy, chances are that the computer will explode before it loses data
> due to media corruption. The calculated time before data loss becomes longer
> than even the pyramids in Egypt could withstand.
These calculations are based on fixed MTBF. But disk MTBF decreases with
age. Most disks are only rated at 3-5 years of expected lifetime. Hence,
use solutions with longer lifetimes (high quality tape = 30 years) and plans
migrating the data to newer media before the expected media lifetime is
In short, if you don't expect to read your 5-year lifetime rated disk for
another 5 years,
then your solution is uhmm... shall we say... in need of improvement.
> The situation becomes similar to having a house with a heavy front door with
> three deadbolt locks, and many glass windows. The front door with its three
> locks is no longer a concern when you are evaluating your home for its
> security against burglary or home invasion because the glass windows are so
> fragile and easily broken.
> It is necessary to look at all the factors which might result in data loss
> before deciding what the most effective steps are to minimize the probability
> of loss.
Yep... and manage the data over time. There is a good reason why library
will never worry about the future of their profession :-)
ZFS storage and performance consulting at http://www.RichardElling.com
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