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MyRocks:recommended configuration for DB of several TB ?



We have started to evaluate MyRocks 3 months ago, our DBs were initially empty and now they have reached 500 GB on-disk. We experience severe performance issues, and we have the feelings it could be better.

https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide is interesting, especially the "Universal Compaction" section:
« Universal compaction has a size limitation. Please be careful when your DB (or column family) size is over 100GB. »

I don't think that write amplification is a bottleneck for our application, so my obvious first question is:
 What compaction style is used in MariaDB and how to make sure that Level Style Compaction is used ?

Also, we haven't set any 'rocksdb_*' option. The default configuration may not be appropriate in our case. For example, rocksdb_block_cache_size (default: 512MB) seems to be one of the most important options, similarly to innodb_buffer_pool_size and tokudb_cache_size, although I have never understood why (see my unanswered question at https://lists.launchpad.net/maria-discuss/msg02683.html ). Although the DB is much bigger than RAM (128 GB), RAM should be big enough to contain the active working set.


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