Page History

...

• Memtable: Memtables are in-memory write buffers. Each new key-value pair is first written to a memtable and each read looks first into the memtable before it looks on disk. Once a memtable is full it becomes immutable and it is replaced by a new memtable. A background thread flushes a memtable asynchronously to disk. Additionally, memtables can also be flushed manually. RocksDB keeps in memory the currently active memtables, full but not yet flushed memtables, and flushed memtables that are kept around to maintain write history in memory.
• Compaction: From time to time RocksDB needs to clean up the data it stores on disk and bring is LSM tree into a good shape (see https://github.com/facebook/rocksdb/wiki/Compaction). Compactions might block writes and flushes. Additionally, RocksDB offers different compaction algorithms with different properties. Thus, it is a good practise to monitor compactions in RocksDB.
• SST files: SST files are the files in which RocksDB stores the data on disk. SST stands for Sorted Sequence Table.
• Version: A version consists of all the live SST files at one point of time. Once a flush or compaction finishes, a new version is created because the list of live SST files has changed. An old version can be used by on-going read requests or compaction jobs. Old versions will eventually be garbage collected.
• Cache: RocksDB caches data in memory for reads. By default, those caches contain only data blocks, i.e., uncompressed sequences of key-value pairs in sorted order. Therefore this cache is often referred to as block cache. However, users can configure RocksDB to also store index and filter blocks in the cache.

The names of the metrics are taken from the following list in the RocksDB repo (with "rocksdb." prefix ripped off):
https://github.com/facebook/rocksdb/blob/b9a4a10659969c71e6f6eab4e4bae8c36ede919f/include/rocksdb/db.h#L654-L686.

Those are public RocksDB properties. We decided to keep the RocksDB names to avoid a mapping that users need to look up or to memorize.

num-immutable-mem-table

Number of immutable memtables that have not yet been flushed. For segmented state stores, the sum of the number of immutable memtables over all segments is reported.

...