Why are there two RAID5 virtual disks in RAID 5?

Question

RAID

There is a storage system Lenovo (IBM) V3700 (two nodes) with 15 disks of 1TB each.

I collect RAID5 from all disks (1 reserve)
The system makes two msidk1(0) with RAID5, 7 disks per 6.5TB each, and unites the whole thing into a 13TB pool.

1. What is the hidden meaning of such a RAID device? Is it right?
2. How is this scheme in terms of speed and reliability compared to just RAID5 of all disks (1 reserve)?
3. What is the preferred RAID for virtual machines?

Answer 1

How is this scheme in terms of speed and reliability compared to just RAID5 of all disks

It is better in terms of speed, because. recording can be simply parallelized. According to reading - it is necessary to measure, it should not be worse.
Reliability is better:
raid5 will survive the loss of any one disk from the array - but only one. Two - the collapse of the array with all the data.
Two raid5s combined into a logical disk are guaranteed to survive the death of any one disk, with a certain amount of luck they will survive the death of the second disk (if it fell off from another array).
With a large number of disks (IMHO, more than 6 disks), it is better to use RAID6, which will survive the death of any two disks in the array, and in terms of performance it is a classmate of RAID5. Especially since you still decide to keep one disk hotspare (which will not help until the array is rebuilt - for terabyte volumes, 2-4 hours of complete utilization of the added disk and a very increased risk of death of one of the other disks of the array along with the entire array) . Or a pair of RAID6 in a logical group, which is even more reliable.
RAID10. Because gives intelligible read/write performance.