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.

{
    
    string strWarning = GetWarnings("rpc");
    if (strWarning != "" && !GetBoolArg("-disablesafemode", DEFAULT_DISABLE_SAFEMODE) &&
        !cmd.okSafeMode)
        throw JSONRPCError(RPC_FORBIDDEN_BY_SAFE_MODE, string("Safe mode: ") + strWarning);
}
std::string HelpMessage(HelpMessageMode mode)
{
    const bool showDebug = GetBoolArg("-help-debug", false);
    
    
    string strUsage = HelpMessageGroup(_("Options:"));
    strUsage += HelpMessageOpt("-?", _("Print this help message and exit"));
    strUsage += HelpMessageOpt("-version", _("Print version and exit"));
    strUsage += HelpMessageOpt("-alertnotify=<cmd>", _("Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)"));
    strUsage += HelpMessageOpt("-blocknotify=<cmd>", _("Execute command when the best block changes (%s in cmd is replaced by block hash)"));
    if (showDebug)
        strUsage += HelpMessageOpt("-blocksonly", strprintf(_("Whether to operate in a blocks only mode (default: %u)"), DEFAULT_BLOCKSONLY));
    strUsage += HelpMessageOpt("-checkblocks=<n>", strprintf(_("How many blocks to check at startup (default: %u, 0 = all)"), DEFAULT_CHECKBLOCKS));
    strUsage += HelpMessageOpt("-checklevel=<n>", strprintf(_("How thorough the block verification of -checkblocks is (0-4, default: %u)"), DEFAULT_CHECKLEVEL));
    strUsage += HelpMessageOpt("-conf=<file>", strprintf(_("Specify configuration file (default: %s)"), BITCOIN_CONF_FILENAME));
    if (mode == HMM_BITCOIND)
    {
#if HAVE_DECL_DAEMON
        strUsage += HelpMessageOpt("-daemon", _("Run in the background as a daemon and accept commands"));
#endif
    }
    strUsage += HelpMessageOpt("-datadir=<dir>", _("Specify data directory"));
    strUsage += HelpMessageOpt("-dbcache=<n>", strprintf(_("Set database cache size in megabytes (%d to %d, default: %d)"), nMinDbCache, nMaxDbCache, nDefaultDbCache));