What is disk cache? What is it used for?

Answer 1

This page has a few things you can affect to "test I/O performance in the real world", which (if you have a Linux system) may help you get an intuitive understanding of what's happening.
https://www.turnkeylinux.org/blog/invalidating-disk-cache

Answer 2

This a part of system which uses write to disk operations. Write to disk is costy operation and disk cache is an RAM-based structure to hold data before writing to disk. Some operation could be done in the fast cache without interaction with slow HDD or fragile SSD.

Answer 3

for everything

Answer 4

Thanks, but I have one more question. If some operation will be done using disk cache without using HDD, will the data wrote on the hdd after that or not if we do not need data afterwards?