Nov 6th, 2012
Choosing a RAID Level by Drive Count
In addition to all other factors, the number of drives available to you plays a significant role in choosing what RAID level is appropriate for you. Ideally RAID is chosen ahead of time in conjunction with chassis and drives in a holistic approach so that the entire system is engineered for the desired purpose, but even in these cases, knowing how drive counts can affect useful RAID choices can be very helpful.
To simplify the list, RAID 0 will be left off of it. RAID 0 is a viable choice for certain niche business scenarios in any count of drives. So there is no need to display it on the list. Also, the list assumes that a hot spare, if it exists, is not included in the count as that is “outside” of the RAID array and so would not be a part of the array drive count.
2 Drives: RAID 1
3 Drives: RAID 1 *
4 Drives: RAID 10
5 Drives: RAID 6
6 Drives: RAID 6 or RAID 10
7 Drives: RAID 6 or RAID 7
8 Drives: RAID 6 or RAID 7 or RAID 10 **
9 Drives: RAID 6 or RAID 7
10 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61
11 Drives: RAID 6 or RAID 7
12 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61
13 Drives: RAID 6 or RAID 7
14 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61or RAID 70/71
15 Drives: RAID 6 or RAID 7 or RAID 60
16 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61 or RAID 70/71
17 Drives: RAID 6 or RAID 7
18 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61 or RAID 70/71
19 Drives: RAID 6 or RAID 7
20 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61 or RAID 70/71
21 Drives: RAID 6 or RAID 7 or RAID 60 or RAID 70
22 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61 or RAID 70/71
23 Drives: RAID 6 or RAID 7
24 Drives: RAID 6 or RAID 7 or RAID 10 or RAID 60/61 or RAID 70/71
25 Drives: RAID 6 or RAID 7 or RAID 60
………
* RAID 1 is technically viable at any drive count of two or more. I have included it only up to three drives because using it beyond that point is generally considered absurd and is completely unheard of in the real world. But technically it would continue to provide equal write performance while continuing to increase in read performance and reliability as more drives are added to the mirror. But for reasons of practicality I have included it only twice on the list where it would actually be useful.
** At six drives and higher both RAID 6 and RAID 10 are viable options for arrays of even drive counts and RAID 6 alone is a viable option for odd numbered drive array counts.
For this list I have only considered the standard RAID levels of 0, 1, 4, 5, 6 and 10. I left 0 off of the list because it is always viable for certain use cases. RAID 5 never appears because there is no time on spindle hard drives today that it should be used, as RAID 5 is an enhancement of RAID 4, it too does not appear on the list. Non-standard double parity RAID solutions such as Netapp’s RAID-DP and Oracle’s RAIDZ2 can be treated as derivations of RAID 6 and apply accordingly. Oracle’s triple parity RAIDZ3 (sometimes called RAID 7) would apply at seven drives and higher but is a non-standard level and extremely rare so I included it in italics.
More commonly, RAID 6 makes sense at six drives or more and RAID 7 at eight drives or more.
Like RAID 4 and 5, RAID levels based on them (RAID 40, 50, 41, 51, 55, etc.) are not appropriate any longer due to the failure and fragility modes of spindle-based hard drives. Complex RAID levels based on RAID 6 and 7 (60, 61, 70, 71, etc.) have a place but are exceedingly rare as they generally have very little cost savings compared to RAID 10 but suffer from performance issues and increased risk. RAID 61 and 71 are almost exclusively effective when the highest order RAID, the mirror component, is over a network rather than local on the system.
