Re: [BackupPC-users] How to use backuppc with TWO HDD

Skip Guenter wrote:
> On Tue, 2009-06-02 at 16:36 +1000, Adam Goryachev wrote:
>   
>> So, using 4 x 100G drives provides 133G usable storage... we can lose
>> any two drives without any data loss. However, from my calculations
>> (which might be wrong), RAID6 would be more efficient. On a 4 drive 100G
>> system you get 200G available storage, and can lose any two drives
>> without data loss.
>>     
>
> So isn't this the same as RAID10 w/ 4 drives, 200GB and can lose 2
> drives (as long as they aren't on the same mirror) and no risk of
> corrupted parity blocks?

With RAID 10, if you loose a drive AND it's mirror, your array is 
toast.  While you CAN loose two drives from a RAID 10 array, they have 
to be specific drives.  With RAID 6 you have X data disks and 2 parity 
disks*.  You can loose ANY two disks from a RAID 6 array without data 
loss.  As you add more disks to a RAID 10 array, you have to dedicate 
half of them to mirroring.  With a RAID 6 array, you only need two 
parity disks.  Any more you add are usable to expand the capacity.

Chris

*This statement is simplified, in that neither RAID 5 nor RAID 6 
actually dedicate a spindle (or two) to parity, but interleave it with 
the data.  RAID 3 has a dedicated parity disk, but doesn't get much 
attention.  With RAID 6, if you loose a disk, performance will not 
suffer, as no data is missing.  RAID 5 down one disk and RAID 6 down two 
disks looses either a part of the data (which is calculated from the 
parity data) or the parity data.

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