NFSv4 Failover

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2020-09-14 11:35

NFS Version 4 has removed support for UDP as an underlying transport protocol (as opposed to V3 which supported both UDP and TCP), therefore all NFSV4 connections are TCP based. This exclusive use of TCP in NFSv4 has implications for failover recovery time in certain scenarios due to TCP's TIME_WAIT (sometimes referred to as 2MSL) state that can be entered into during multiple failover operations.

The reason for a TCP socket to enter a TIME_WAIT state is to prevent delayed data packets from one connection being misinterpreted as part of a subsequent newly established connection on the same machine (applying stale data from a previous connection to the current one could have potentially disastrous affects on data integrity).

The implication of TIME_WAIT for failover is observed when HA services are moved from one node to another and then back again in a short period of time. Once the initial move is complete the originating server enters the TIME_WAIT state as part of the normal TCP protocol. If during this 'wait period' services are moved back to the originating server clients will be unable to re-connect until the TIME_WAIT period has expired (and in some cases the client connections will timeout themselves), therefore manual moves back and forth in quick succession (circa 2-4 minutes) between machines that provide data over NFSv4 should be avoided. This type of quick failover/failback scenario is only normally seen during system testing exercises and not representative of production environments.

For machines where the failover was instigated as a result of a systems crash, TIME_WAIT is irrelevant as the TCP connections will have no knowledge of the previous connection.

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