When running an active node (BFT leader or stake pool) it is interesting to be able to make choices on how to manage the pending transactions: how long to keep them, how to prioritize them etc.
mempool field in your node config file is not mandatory, by default it is set
mempool: pool_max_entries: 10000 log_max_entries: 100000
pool_max_entries: (optional, default is 10000). Set a maximum size of the mempool
log_max_entries: (optional, default is 100000). Set a maximum size of fragment logs
persistent_log: (optional, disabled by default) log all incoming fragments to log files, rotated on a hourly basis. The value is an object, with the
dirfield specifying the directory name where log files are stored.
A persistent log is a collection of records comprised of a UNIX timestamp of when a fragment was registereed by the mempool followed by the hex-encoded fragment body. This log is a line-delimited JSON stream.
Keep in mind that enabling persistent logs could result in impaired performance of the node if disk operations are slow. Consider using a reasonably fast ssd for best results.