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.

4 thoughts on “Choosing a RAID Level by Drive Count

  1. SAM,

    You seem to be basing your generic RAID configuration guidelines based on spindle/device/drive count and space, perhaps with an inferred availability aspect. However while general and generic, people tend to take these out of context and as you probably know, there is more than just space capacity as there is also performance and availability among other considerations to what RAID level to use when/where.

    Using generic rules of thumb can be a starting point, however what about performance of reads vs. writes, random vs. sequential, large vs. small application usage profiles, where are those reflected in your guidelines?

    Also, I know you don’t like RAID 5 and while I use it for 4 drive configs including with large TB+ sized drives, they are also backed-up and protected, however I also would be the first to tell you that RAID 5 is not good for everything.

    However I also use RAID 1, RAID 6 among others in different ways depending on the need, this includes hardware/system/adapter as well as software based.

    Just like RAID 10 or RAID 1 or RAID 6 or other RAID levels and permutations are not applicable for everything, it all depends on the application needs, requirements which also means more than just based on number of drives.

    Btw, RAID 5 is more than a slight enhancement over RAID 4 (dedicated parity), as with RAID 5, there is no single dedicated priority drive with each drive taking turns handling the parity information, and with RAID 6, the dual parity is likewise spread across the drives in a manner where all drives help out (RAID 4 Dual Parity otoh have just that, two drives dedicated to parity)

    Here’s a RAID refreseher

    http://www.infostor.com/disk-arrays/raid/raid-remains-relevant-really-1.html

    Of course, if you are bound by what a particular system or set of hardware or software that you may have can do, that will have an impact on what you also do…

    Cheers gs

    Greg Schulz @Storageio

  2. Wouldn’t raid 50 make sense by striping across several smaller raid5 pools, whereby rebuild time in event of drive failure would only effect one pool, and a limited number of drives necessary to involve in a rebuild?

  3. Yes RAID 50 does what you say by reducing the failure pool, but it does it in a very inefficient way. RAID 6 reduces the risk even more than RAID 50 does while costing less. So RAID 50 is eliminated by always being inferior to RAID 6. Plus even “small” RAID 5 sets today are generally reckless, so there is no efficient means of making a good RAID 50. The nature of RAID 5 is that it never makes sense in URE sensititve (read: spinning rust”) disks so, by extension, RAID 50 is just a collection on interdependent risky arrays making it bad as well. Not “as” bad as the same array with a single RAID 5, but not better than RAID 6 or something else in the same array setup.

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