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Re: [zfs-discuss] Performance drop during scrub?

Subject: Re: [zfs-discuss] Performance drop during scrub?
From: Richard Elling
Date: Sun, 2 May 2010 12:12:07 -0700
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 :-)

 -- richard

ZFS storage and performance consulting at http://www.RichardElling.com

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