RAID 0+1

[Benjamin Franz]

On Tue, 21 Mar 2006, Gordon Messmer wrote:

> Jeff Vian wrote:
>> Why not raid 5?  The available space is n-1 for the number of drives
>> used.
>
> Read and write performance are both significantly better under RAID 10.  The 
> only reason to use RAID 5 over RAID 10 is cost.  If you can't afford (or 
> justify) RAID 10, then RAID 5 is a lower-cost, lower-performance option.
>
>> A single disk problem takes out the entire mirror
>> copy (3 disks) because of striping.
>
> If you strip sets of RAID 1 devices, then you'll only have to rebuild the 
> disk that fails, when it's replaced.
>
>> A data error on one copy and any
>> other error (even in a different location) on the second copy will take
>> everything out (again because of the striping effect of raid 0). 
>
> Simultaneous errors on multiple drives will take out pretty much any RAID 
> setup.

RAID10 has a statistical resistance to multiple simultaneous failures.

With six drives, RAID10 can withstand 1 failed drive guaranteed, 2 failed 
drives with an 80% probability and 3 simultaneously failed drives with a 
probability of 40%. Even with only 4 drives in a RAID10 array, you still 
get a 67% chance of surviving a two drive failure situation. RAID10 is 
noticably more robust than RAID5 - which can never withstand more than one 
simultaneous drive failure.

>> The same 6 drives in raid 5 would give you 5*73 or 365gb of space and a
>> single drive failure would not in any way harm your data. The redundant
>> drive feature would keep everything working while the one drive is
>> replaced.
>
> But a data error on one drive, and any other error on another drive will 
> prevent you from rebuilding the array, possibly destroying the entire thing. 
> Alarmist, isn't it?
>
> RAID is not a replacement for backups.

Very true. On and offsite.

-- 
Benjamin Franz

If you can't handle reality, it *will* handle you.