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.