Jul
09

Making A Better Distributed System – Nutanix Degraded Node Detection

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Distributed systems are hard, there no doubt about that. One of the major problems is what to do when a node is unhealthy and can be affecting performance of the overall cluster. Fail hard, fail fast is distributed system principle but how do you go about detecting an issue before even a failure occurs? AOS 4.5.3, 4.6.2 and 4.7 will includes the Nutanix implementation of degraded node detection and isolation. A bad performing hardware component or network issue can be a death of thousands cuts versus a failure which is pretty cut and dry. If a remote CVM is not performing well it can affect the acknowledgement of writes coming from other hosts and other factors may affect performance like:

* Significant network bandwidth reduction
* Network packet drops
* CPU Soft lockups
* Partially bad disks
* Hardware issues

The list of issues can even be unknown so Nutanix Engineering has come with a score systems that uses votes to make sure everything can be compared.
Services running on each node of the cluster will publish scores/votes for services running on other nodes. Peer health scores will be computed based on various metrics like RPC latency, RPC failures/timeouts, Network latency etc. If services running on one node are consistently receiving bad scores for large period (~10 mins), then other peers will convict that node as degraded node.

Walk, Crawl, Run – Degraded Node Expectations:
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A node will not be marked as degraded if current cluster Fault Tolerance (FT) level is less than desired value. Upgrades and break fix actions will not be allowed while a node is in the degraded state. A node will only be marked as degraded if we get bad peer health scores for 10 minutes. In AOS 4.5.3, the first shipping AOS release to include this feature, the default settings are that degraded node logging will be enabled but degraded node action will be disabled. In AOS 4.7 and AOS 4.6.2 additional user controls will be provided to select an “action policy” for when a degraded node is detected. Options should include No Action, Reboot CVM or Shutdown Node). While the peers scoring is always on, the action is side is disabled for the first release as ultra conservative approach.

In AOS 4.5.3 if the degraded node action setting is enabled leadership of critical services will not be hosted on the degraded node. A degraded node will be put into maintenance mode and CVM will be rebooted. Services will not start on this CVM upon reboot. An Alert will be generated for degraded node.

In AOS 4.7 and AOS 4.6.2 additional user controls will be provided to select an “action policy” for when a degraded node is detected. Options should include No Action, Reboot CVM or Shutdown Node

To enable the degraded node action setting use the NCLI command:

nutanix@cvm:~$ ncli cluster edit-params disable-degraded-node-monitoring=false

The feature will further increase the availability and resilience for Nutanix customers. While top performance numbers grab the headlines, remember the first step is to have a running cluster.

AI for the control plane………… Maybe we’ll get out voted for our jobs!

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