[AccessD] Google's Disk Failure Experience

jwcolby jwcolby at colbyconsulting.com
Tue Jul 20 13:27:29 CDT 2010


 >How much is your time worth?

This is a classic case of insufficient analysis...

The cost is not the cost of the drive.  It is the cost of the drive (twice the drives), plus the 
cost of the SATA ports (twice) plus the cost of the power supply (twice the disk current - 12v), 
plus the cost of electricity to run the disks (twice the electricity) plus the cost of a case big 
enough to handle enough drives (twice as large disk cage) plus...

Of course all my "twices" are on a "per disk used" basis, not a total system cost and I understand 
the difference.

Furthermore, my time isn't the cost of my time to rip 200 dvds, it is the cost of my time to rip 200 
dvds divided by the probability of losing 2 disks at the same time.

I ripped my drives as I wanted to watch them, so it was an incremental cost accumulated over time. 
The actual time is about 2 minutes per dvd.  200 dvds per drive is an actual cost of 200 minutes to 
fill a drive.  Would I actually rerip all of the disks?  No because many of them are for my kids who 
are now older and don't even watch those disks any more, not to mention the disks I ripped that I 
just don't care about.

HOWEVER... In the two years that I have been using the system, I have never lost one of these 
drives.  Thus my time to "do it over again" is zero (so far).  I have been working with raid 
extensively for about 5 years, and in those 5 years I have lost single drives but I have never lost 
two drives at the same time.

I am sure that there are numbers out there that discuss the probability of two drive failures.  In 
fact IIRC from that paper by google, the probability of failure of any given single drive over 5 
years was about 14%?  The probability of two failures (my simple math) would be .14 * .14 = .0196 or 
~ 2%.  That is for two failures, NOT two SIMULTANEOUS failures.

All of this matters if the cost is catastrophic.  Facing a 2% chance that I will have to re-rip 200 
dvds in any given 5 year period, I am unwilling to commit the extra money to preventing this 
possibility.

This whole discussion does point out that an analysis of the actual numbers might cause one to come 
to a different conclusion.  And who knows, you might decide "screw the costs", it is worth it to me.

It is not worth it to me.

Look at Unraid.

http://lime-technology.com/

I am not trying to sell anyone on unRaid, I am simply saying consider it for a specific class of 
redundancy needs.  For this level of redundancy need, unRaid seems like a good compromise.

John W. Colby
www.ColbyConsulting.com


Michael Bahr wrote:
> John, for your media collections you really should go only Raid 1.  Yeah
> if you lose the HD you can re-do everything but who has the time?  Besides
> 2TB HD's are ~<$150 or so.  How much is your time worth?
> 
> Mike...
> 
>> I am about to build an UnRaid for my massive video / music collection.  I
>> currently have no
>> protection on that so if I lose a disk I lose all of that on one disk.  With Unraid I
>> would have "raid 5 like"
>> storage so that in the event of single drive failure I can still recover.
>> If I do lose it I just
>> re-rip.  Not the end of the world but not something I want to do.
>>
>> John W. Colby
>> www.ColbyConsulting.com




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