L1 and Ln cache: when are they written?
1) Usually any accessed data are saved into L1, independent whether they were previously available in L1, L2... or memory only.

For exclusive cache, data are stored ONLY in L1, for inclusive one, in the entire hierarchy. There are also mixed strategies. Also, some CPUs have "victim cache", which is extra cache level which gets ONLY data moved out of LLC (last level cache).

2) Yes, for write-back exclusive cache.

3) You misunderstood his explanation. He said that data are fetched and placed to both caches. Inclusion, btw, isn't the "bit set", but entire cache policy. The policy is built right into the hardware.

4) Exclusive cache better use precious space, but each time you have to bring new block into L1, you have to displace some block to L2, then some one to L3... and finally some one to memory. This means a lot of work. With inclusive cache, you just drop data from L1 cache, or replace older copy in L2 with newer copy displaced from L1.

As result, exclusive strategy is more optimal when adjacent cache levels has closer sizes (f.e. &lt;=4x) and inclusive is better when sizes are very different.

More info at en.wikipedia.org/wiki/Cache_inclusion_policy