Re: Triple parity and beyond

On 22/11/13 09:38, Stan Hoeppner wrote:

On 11/21/2013 3:07 AM, David Brown wrote:

quoted

For example, with 20 disks at 1 TB each, you can have:

All correct, and these are maximum redundancies.

Maximum:

quoted

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:

quoted

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.