ZFS ARC size vs target size

Well, I am some years late, but I hope that this can help others.

• c is the target ARC size. It is 50% of total RAM by default, but it can shrink on demand if the system is under memory pressure
• size is the current ARC size. Given enough time, it will tend to c
• data_size is the size of cached user data. It does not include metadata (and this is the reason why it can be significantly smaller than MRU and/or MFU when facing metadata-heavy workload)
• metadata_size is the size of cached metadata
• hdr_size is the size of L2ARC headers stored in main ARC
• overhead_size is the size of various buffer which are allocated when decompressing dentries