In the Substrate CLI why can we limit the memory that the RocksDB database cache can use if we choose to use it here, but we're not given the option to limit the memory that ParityDB uses if we choose that database instead here?

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After further investigation in this question and answer, I discovered that both RocksDB and ParityDB are both persistent key-value (KV) stores, but that ParityDB uses "OS page cache" instead of "custom data caching", where it states in the Substrate Documentation here that they use key-value cache because "accessing trie data is costly". In the ParityDB README here it states that the ParityDB "Database does not implement any custom data caching. Instead, it relies on OS page cache. Performance of a large database therefore depends on how much system memory is available to be used in OS page cache."

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