I know nothing about VMs or filesystems but I absolutely enjoyed this article. The language was very clear and easy to follow. Would be following the blog from now on.

I have a question about the copy-on-write example involving VM A and VM B. It says t VM B will directly use all the data from VM A and for any change, it copies the block, writes into it and reads from it after this.

But what if, say, block 2 is changed by VM A and was never written to by VM B? Wouldn't VM B read the changed block 2? Clearly, it doesn't happen cause a fork is a copy, but an explanation of how this is tackled is appreciated!