Definition
Mempool persistence is the feature that lets a Bitcoin Core node remember its pool of unconfirmed transactions across a restart. On a clean shutdown the node serializes the current memory pool to a file named mempool.dat in the data directory, and on the next startup it reads that file back and re-loads the transactions, re-checking each one against current consensus and policy rules. Without this, every restart would empty the mempool, and the node would have to slowly relearn pending transactions from its peers.
Why it matters
A warm, well-populated mempool improves fee estimation, gives miners and wallets a realistic view of the fee market immediately after boot, and reduces redundant relay traffic. The getmempoolinfo RPC exposes a loaded flag that is false until the node has finished restoring mempool.dat, so tools can wait for a complete picture before trusting fee data. Persistence is controlled by the -persistmempool option, which is on by default, and the file can also be written on demand with the savemempool RPC.
Limits and caveats
Persistence only applies to a clean shutdown; a crash or power loss may leave a stale or missing file, in which case the node simply starts with an empty mempool and refills from peers. Transactions that no longer meet the current mempool minimum fee or have since confirmed or conflicted are dropped during reload.
For related node behavior, see mempool minimum fee and dbcache.
In Simple Terms
Mempool persistence is the feature that lets a Bitcoin Core node remember its pool of unconfirmed transactions across a restart. On a clean shutdown the…