cache_metadata_size size varaible questions

[pahome]

I nearly read cache like tools and read the cache_metadata_size.cc with some questions.

uint64_t
cache_metadata_size_cmd::calc_size(uint64_t nr_blocks, uint32_t max_hint_width)
{
	uint64_t const SECTOR_SIZE = 512;
	uint64_t const TRANSACTION_OVERHEAD = meg(4);
	uint64_t const BYTES_PER_BLOCK = 16;
	uint64_t const HINT_OVERHEAD_PER_BLOCK = 8;

I want to ask how to estimate the BYTES_PER_BLOCK & HINT_OVERHEAD_PER_BLOCK ?

I thought BYTES_PER_BLOCK be 12 ? because the following struct in mapping_array.h

struct mapping {
		uint64_t oblock_;
		uint32_t flags_;
	};

And what's the HINT_OVERHEAD_PER_BLOCK meaning?

thanks.

[mingnus]

The equation in cache_metadata_size assumes the mappings and hints are stored in btrees rather than arrays: The BYTES_PER_BLOCK denotes the 16-byte key & value for each cache block, and the HINT_OVERHEAD_PER_BLOCK stands for the 8-byte key for each hint entry. It's an outdated design and will be revised someday.