meta server disk-based storage.

[meijies]

Currently, meta server use disk-based commit logs, and apply these logs to memory data structure. But as time goes on, too much commit logs stored at disk, when we needed to restart meta server, it's need long time to read the commit log to recover the memory data structure. and disk space also is wasted.

[jovany-wang]

Did you mean that supporting checkpoint for state machine of meta server?