Why does the file copy speed fluctuate?

Question

RAID

I use hardware Microsemi Adaptec RAID 8405E V2. Debian 10 is installed, on which Proxmox 6 is deployed.
When I just copied the file with the cp command on the command line , I noticed that the speed constantly fluctuates, then 20 MB / sec, then 400 MB / sec.
As far as I remember, in the RAID controller settings, I turned on the caching setting, sort of. Maybe because of her? What parameters of such a RAID controller should be paid attention to in order to increase the write speed? So that the speed does not fluctuate so much during normal copying?
Copies a 150 GB file for about 15 minutes ... And I copy it to the same directory where the source file itself is located.
Raid mirror (RAID 1)

Answer 1

Buffer full, buffer flush, buffer full, buffer flush.

Answer 2

You need to look at what kind of disks. But most likely this is indeed the behavior of the cache, without it, most likely, in this scenario, performance will drop, but it will not jump.
10GB per minute, while you have both reading and writing at the same time, this is about 165mb / s, it depends a lot or a little on disks and file fragmentation. For non-ssd, this is very decent, for ssd it seems to be small, but as I have already indicated, there are many nuances.