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. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
