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RAID 55

Why is a RAID 55 configuration never used as a RAID configuration?
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I guess, it's because it's too expensive, and has no advantage regarding speed. So, most of the time, i guess, RAID6 is chosen in such requirements.
Depending on the application that you apply your question to?
RAID 6 is usually the better choice.
For further reading on RAID Arrays, please read:
docs.oracle.com
Link:
https://docs.oracle.com/cd/E19494-01/820-1260-15/appendixf.html#50548797_61540
Distinguished Expert 2019
Commented:
It would be exceedingly slow for random write.

For RAID 5 there are 4 physical IOPS per logical write; read data and parity (then XOR with new data), then write data and parity. (XOR isn't an I/O so I put in brackets).

Now let's have RAID 55 with 6 disks per parity group...
6 reads to get the data, 6 reads to get the parity, (XOR) 6 writes for the new data and 6 writes for the new parity = 24 I/Os to do a single write I/O. RAID 6 or 60 would only need 6 I/Os to do that same work.

There is a nested RAID level called RAID 53, but even that isn't RAID 5+3, it's RAID 0+3 and exceedingly rare to see used.  There are however instances where such esoteric RAID levels exist without being deliberately constructed, underlying hardware may have inbuilt striping such as fast NAND flash which can have an internal RAID0 stripe across several chips. HP LeftHand also does Network RAID 5 which is RAID 5 over multiple storage nodes so that one whole node can die and the data is still available and if RAID 5 is used on each node that's in effect RAID 55 except that Network RAID 5 uses background parity calculation instead of doing it on the fly.

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Commented:
andyalder,

What do you mean by Network RAID 5 is this a special RAID 5 implemented by HP Lefthand SANs?
Distinguished Expert 2019

Commented:
Yes, Network RAID is a RAID array made of multiple storage nodes, it's relatively simple in concept, take two servers with storage on them, mirror the two servers and that's Network RAID 1.

Author

Commented:
andyalder,

So if I were to buy a HP LeftHand P4300 G2 SAN for example (http://h20564.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c03738427) consisting of two racks of Eight disks. I could then configure it as a RAID 5 SAN. So I could loose one disk from each rack without loss of data. If I were then to buy another P4300 this would give me four racks of eight disks. Could I then configure each of the first three racks as RAID 5. Then configure the racks as RAID five so the first three racks hold the data and the fourth rack is a parity rack. So I could loose an entire rack and still have my data?
Distinguished Expert 2019

Commented:
Yes. It was just one example of two RAID5s nested. As I said it's not used much and it's not true RAID55 as the replication is asynchronous. You can of course get 4 RAID controllers with external disks and put them in one machine and do your own nested hardware RAID 5 + software RAID 5.

As others have already mentioned though nobody offers true RAID55 because RAID 6 or 60 is better.