Re: Triple parity and beyond

[David Brown <david.brown@hesbynett.no>]

On 22/11/13 09:38, Stan Hoeppner wrote:
> On 11/21/2013 3:07 AM, David Brown wrote:
> 
>> For example, with 20 disks at 1 TB each, you can have:
> 
> All correct, and these are maximum redundancies.
> 
> Maximum:
> 
>> raid5 = 19TB, 1 disk redundancy
>> raid6 = 18TB, 2 disk redundancy
>> raid6.3 = 17TB, 3 disk redundancy
>> raid6.4 = 16TB, 4 disk redundancy
>> raid6.5 = 15TB, 5 disk redundancy
> 
> 
> These are not fully correct, because only the minimums are stated.  With
> any mirror based array one can lose half the disks as long as no two are
> in one mirror.  The probability of a pair failing together is very low,
> and this probability decreases even further as the number of drives in
> the array increases.  This is one of the many reasons RAID 10 has been
> so popular for so many years.
> 
> Minimum:
> 
>> raid10 = 10TB, 1 disk redundancy
>> raid15 = 8TB, 3 disk redundancy
>> raid16 = 6TB, 5 disk redundancy
> 
> Maximum:
> 
> RAID 10 = 10 disk redundancy
> RAID 15 = 11 disk redundancy

12 disks maximum (you have 8 with data, the rest are mirrors, parity, or
mirrors of parity).

> RAID 16 = 12 disk redundancy

14 disks maximum (you have 6 with data, the rest are mirrors, parity, or
mirrors of parity).


> 
> Range:
> 
> RAID 10 = 1-10 disk redundancy
> RAID 15 = 3-11 disk redundancy
> RAID 16 = 5-12 disk redundancy
> 
> 

Yes, I know these are the minimum redundancies.  But that's a vital
figure for reliability (even if the range is important for statistical
averages).  When one disk in a raid10 array fails, your main concern is
about failures or URE's in the other half of the pair - it doesn't help
to know that another nine disks can "safely" fail too.