RAID5 vs RAID4 purpose of floating parity

I still did not get why is RAID5 better than RAID4. I understand both computes parity bits that are used for recovering if some failure occurs, the only difference is in storing those parity bits. I have borrowed diagrams from here How does parity work on a RAID-5 array

A B (A XOR B) 0 0 0 1 1 0 0 1 1 1 0 1 

RAID4

Disk1 Disk2 Disk3 Disk4 ---------------------------- data1 data1 data1 parity1 data2 data2 data2 parity2 data3 data3 data3 parity3 data4 data4 data4 parity4 

Lets say that first row is:

data1 = 1 data1 = 0 data1 = 1 parity1 = 0 (COMPUTED: 1 XOR 0 XOR 1 = 0) 

RAID5

Disk1 Disk2 Disk3 Disk4 ---------------------------- parity1 data1 data1 data1 data2 parity2 data2 data2 data3 data3 parity3 data3 data4 data4 data4 parity4 

Lets say that first row is:

parity1 = 0 (COMPUTED: 1 XOR 0 XOR 1 = 0) data1 = 1 data1 = 0 data1 = 1 

Scanarios:

1. RAID4 - Disk3 FAILURE:

data1 = 1 data1 = 0 data1 = 1 (COMPUTED: 1 XOR 0 XOR 0 = 1) parity1 = 0 

2. RAID4 - Disk4 (parity) FAILURE:

data1 = 1 data1 = 0 data1 = 1 parity1 = 0 (COMPUTED: 1 XOR 0 XOR 1 = 0) 

etc.

In general: when RAID(4 or 5) uses N disks and one fails. I can take all remaining non failed disks (N-1) and XOR (since XOR is associative operation) values and I will get the failed value. What is the benefit of storing parity not on dedicated disk but rather cycle them? Is there some performance benefit or what? Thank you

2 Answers

There is a performance difference in that with RAID 4 each change requires writing to the single parity disk, which means things can queue waiting to update the parity data on that disk.

With RAID 5 you have a significant reduction in this because the parity update load is spread across multiple disks, so there's less chance if getting stuck in a queue.

Here's a nice link from Fujitsu with a short explanation and some nice animations to help clarify the performance/penalties of RAID 4 (as well as other RAID levels).

OWC has said with their enclosures and using their software SoftRaid, that RAID 4 is faster for reads than RAID 5. Same level of protection, same write speeds, better read speeds, I'm going with RAID 4 in my OWC Thunderbolt 3 enclosure with four NVMe drives.