Originally Posted by zicoz
Sure it's replacable but the time needed to replace it outweighs the cost of the extra Storage.
You are making a mistake in determining the value of an additional backup.
One might say that the extra backup only has value if you have a disaster that causes data loss in the primary storage. Say that the probability of such a disaster is P
, then you need to compare the value of your time to replace the data, Vt
with the cost of the extra backup, C
, weighted by 1 / P
. Which can also be written as :P Vt
Which costs more? The value of your time multiplied by the probability of a data-loss disaster? Or the extra backup?
To take an example, say that the probability of a data loss disaster is 10% , which I think is an overestimate for most people who have put together a good snapshot RAID setup with some simple safeguards, but 10% is a nice round number, so let us go with it.
In my case, replacing the data means re-ripping from my blu-rays. But I am working on my computer many hours every week, so ripping blu-rays while I am working only adds about a minute of actual labor per blu-ray, the time to put the disc in the tray, type a name, and hit the start button. I don't count the time while it is ripping, since I am working on other things. So, 1 minute per blu-ray, one hour of my time for every 60 blu-rays that need to be re-ripped.
Let's say I value my time at $50 per hour. But weighted by 10% probability of disaster, that is just $5/hr, or about $0.08 per blu-ray to re-rip. The average size of my blu-ray rips is about 25GB. So that is about $0.0033 per GB of data that was lost. That is the weighted value of my time to replace the data, $0.0033 per GB.
How much does the extra backup cost? For simplicity I will ignore the value of the time taken to do the maintenance to keep the extra backup up to date and running smoothly, and I will ignore the cost of any computer equipment used to manage the backup HDDs, and I will just look at the cost of bare HDDs.
At present, the least expensive 3TB HDD on newegg is $150. The least expensive 2TB is $100. So that is $50 per TB. But lets say that if we waited for a sale and/or bought in bulk, we could get that down to $40 per TB. That comes to $0.040 per GB.
So the cost of the extra backup exceeds 12 times the weighted value of my time to restore the data.
Clearly, the extra backup is not worth the cost, and it is not even a close decision.Edited by jim2100 - 8/11/12 at 10:42am