Currently the mempool isn't persisted across restarts. The historical argument against doing this is that it would make transactions immortal. Expiration now addresses that issue.

Without preserving the mempool node behavior changes drastically across restarts which complicates production use (e.g. mining overly small/low fee blocks) as well as development testing. The lack of retention also causes the loss of txn priority modifications made with prioritisetransaction which makes the API less reliable.

return error("LoadBlockIndex(): writing genesis block to disk failed");
            CBlockIndex *pindex = AddToBlockIndex(block);
            if (!ReceivedBlockTransactions(block, state, pindex, blockPos))
                return error("LoadBlockIndex(): genesis block not accepted");
            
            return FlushStateToDisk(state, FLUSH_STATE_ALWAYS);
        } catch (const std::runtime_error& e) {
            return error("LoadBlockIndex(): failed to initialize block database: %s", e.what());
        }
    }
    return true;
}
bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskBlockPos *dbp)
{
    
    static std::multimap<uint256, CDiskBlockPos> mapBlocksUnknownParent;
    int64_t nStart = GetTimeMillis();
    int nLoaded = 0;
    try {
        
        CBufferedFile blkdat(fileIn, 2*MAX_BLOCK_SERIALIZED_SIZE, MAX_BLOCK_SERIALIZED_SIZE+8, SER_DISK, CLIENT_VERSION);
        uint64_t nRewind = blkdat.GetPos();
        while (!blkdat.eof()) {
            boost::this_thread::interruption_point();
            blkdat.SetPos(nRewind);
            nRewind++; 
            blkdat.SetLimit(); 
            unsigned int nSize = 0;
            try {