Tuning the TCP for Fast Failure Detection

An unplanned failure chiefly takes the form of a node failure or a network failure. In both instances, the network layer retries to connect to the failed node. The number of retry attempts is dictated by the TCP parameter /proc/sys/net/ipv4/tcp_syn_retries. The default of that parameter is 5 (in Linux), resulting in a latency of more than a minute to detect the node failure. The problem is compounded when the same failed node is contacted repeatedly in the context of a long operation, such as when a client accesses multiple data objects present on that node.

MapR stack solves the problem by remembering (caching) the information about a node’s failure, and by not contacting that node for subsequent operations on data objects present on that node. Since all form of data is replicated, MapR services find alternative locations for a data object. This feature is in-built into the current software and does not have to be enabled explicitly. Hence, the communication between a client and a recently failed node incurs a one-time long-duration latency. As mentioned before, that latency is governed by the number of retries at the TCP level. Hence, to further improve the one-time longer latency of an operation between a pair of nodes, it is recommended that the number of TCP retries be decreased from 5 to 4, resulting in a latency of about 30 seconds.

Setting the Timeout for TCP Connections

To set the TCP retry count, set the value of tcp_syn_retries to 4 in the /proc/sys/net/ipv4/ directory. For example:

echo 4 > /proc/sys/net/ipv4/tcp_syn_retries

This TCP setting of 4 ensures that the TCP stack takes about 30 seconds to detect failure of a remote node. To ensure that this setting is persistent across system reboots, set this value in the /etc/sysctl.conf file also.

Warning: This setting impacts all TCP connections to and from a node. Hence, caution must be exercised when lowering this further. Also, in some instances, reducing this further may result in a node being incorrectly flagged as unavailable.