I've noticed some issues a while ago with TCP syn cookies breaking DLM cluster setup of 64 hosts. (63 hosts all trying to join the same cluster generate enough

I've noticed some issues a while ago with TCP syn cookies breaking DLM cluster setup of 64 hosts. (63 hosts all trying to join the same cluster generate enough TCP traffic that the kernel thinks it is a flood and starts sending syn cookies, but then the DLM join of some hosts doesn't actually complete). This can be worked around by increasing the backlog (one of the mitigations listed in this article): --- a/fs/dlm/lowcomms.c +++ b/fs/dlm/lowcomms.c @@ -1209,7 +1209,7 @@ static struct socket *tcp_create_listen_sock(struct connection *con, log_print("Set keepalive failed: %d", result); } - result = sock->ops->listen(sock, 5); + result = sock->ops->listen(sock, 128); This is mostly a theoretical issue, because upstream RHEL used to support only up to 16 hosts in a cluster.