Editing RAID (section)

== Nested (hybrid) RAID ==
{{Main|Nested RAID levels}}

In what was originally termed ''hybrid RAID'',<ref name="Vijayan">{{cite book |last1=Vijayan |first1=S. |last2=Selvamani |first2=S. |last3=Vijayan |first3=S |title=Proceedings of the 1995 International Conference on Parallel Processing: Volume 1 |chapter-url=https://books.google.com/books?id=QliANH5G3_gC&q=%22hybrid+raid%22 |year=1995 |publisher=[[CRC Press]] |isbn=978-0-8493-2615-8 |pages=I–146''ff'' |chapter=Dual-Crosshatch Disk Array: A Highly Reliable Hybrid-RAID Architecture |via=[[Google Books]]}}</ref> many storage controllers allow RAID levels to be nested. The elements of a ''RAID'' may be either individual drives or arrays themselves. Arrays are rarely nested more than one level deep.

The final array is known as the top array. When the top array is RAID&nbsp;0 (such as in RAID&nbsp;1+0 and RAID&nbsp;5+0), most vendors omit the "+" (yielding [[RAID&nbsp;10]] and RAID&nbsp;50, respectively).

* '''RAID&nbsp;0+1:''' creates two stripes and mirrors them. If a single drive failure occurs then one of the mirrors has failed, at this point it is running effectively as RAID 0 with no redundancy. Significantly higher risk is introduced during a rebuild than RAID 1+0 as all the data from all the drives in the remaining stripe has to be read rather than just from one drive, increasing the chance of an unrecoverable read error (URE) and significantly extending the rebuild window.<ref>{{Cite web |url=http://aput.net/~jheiss/raid10/ |title=Why is RAID 1+0 better than RAID 0+1? |website=aput.net |access-date=2016-05-23}}</ref><ref>{{Cite web |url=http://www.thegeekstuff.com/2011/10/raid10-vs-raid01/ |title=RAID 10 Vs RAID 01 (RAID 1+0 Vs RAID 0+1) Explained with Diagram |website=www.thegeekstuff.com |access-date=2016-05-23}}</ref><ref>{{Cite web |url=http://www.smbitjournal.com/2014/07/comparing-raid-10-and-raid-01/ |title=Comparing RAID 10 and RAID 01 {{!}} SMB IT Journal |website=www.smbitjournal.com |date=30 July 2014 |access-date=2016-05-23}}</ref>
* '''RAID&nbsp;1+0:''' (see: [[RAID&nbsp;10]]) creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives.<ref name="layton-lm">Jeffrey B. Layton: {{usurped|1=[https://web.archive.org/web/20110615143527/http://www.linux-mag.com/id/7928/?hq_e=el&hq_m=1151565&hq_l=36&hq_v=3fa9646c7f "Intro to Nested-RAID: RAID-01 and RAID-10"]}}, Linux Magazine, January 6, 2011</ref>
* '''[[JBOD]] RAID N+N:''' With JBOD (''just a bunch of disks''), it is possible to concatenate disks, but also volumes such as RAID sets. With larger drive capacities, write delay and rebuilding time increase dramatically (especially, as described above, with RAID 5 and RAID 6). By splitting a larger RAID N set into smaller subsets and concatenating them with linear JBOD,{{clarify|reason='linear' JBOD hasn't been mentioned here yet|date=June 2018}} write and rebuilding time will be reduced. If a hardware RAID controller is not capable of nesting linear JBOD with RAID N, then linear JBOD can be achieved with OS-level software RAID in combination with separate RAID N subset volumes created within one, or more, hardware RAID controller(s). Besides a drastic speed increase, this also provides a substantial advantage: the possibility to start a linear JBOD with a small set of disks and to be able to expand the total set with disks of different size, later on (in time, disks of bigger size become available on the market). There is another advantage in the form of disaster recovery (if a RAID N subset happens to fail, then the data on the other RAID N subsets is not lost, reducing restore time). {{citation needed|date=July 2017}}