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nodes-cluster-overcommit-resource-requests.adoc

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Understanding resource requests and overcommitment

For each compute resource, a container may specify a resource request and limit. Scheduling decisions are made based on the request to ensure that a node has enough capacity available to meet the requested value. If a container specifies limits, but omits requests, the requests are defaulted to the limits. A container is not able to exceed the specified limit on the node.

The enforcement of limits is dependent upon the compute resource type. If a container makes no request or limit, the container is scheduled to a node with no resource guarantees. In practice, the container is able to consume as much of the specified resource as is available with the lowest local priority. In low resource situations, containers that specify no resource requests are given the lowest quality of service.

Scheduling is based on resources requested, while quota and hard limits refer to resource limits, which can be set higher than requested resources. The difference between request and limit determines the level of overcommit; for instance, if a container is given a memory request of 1Gi and a memory limit of 2Gi, it is scheduled based on the 1Gi request being available on the node, but could use up to 2Gi; so it is 200% overcommitted.

Understanding Buffer Chunk Limiting for Fluentd

If the Fluentd logger is unable to keep up with a high number of logs, it will need to switch to file buffering to reduce memory usage and prevent data loss.

Fluentd file buffering stores records in chunks. Chunks are stored in buffers.

The Fluentd buffer_chunk_limit is determined by the environment variable BUFFER_SIZE_LIMIT, which has the default value 8m. The file buffer size per output is determined by the environment variable FILE_BUFFER_LIMIT, which has the default value 256Mi. The permanent volume size must be larger than FILE_BUFFER_LIMIT multiplied by the output.

On the Fluentd pods, permanent volume /var/lib/fluentd should be prepared by the PVC or hostmount, for example. That area is then used for the file buffers.

The buffer_type and buffer_path are configured in the Fluentd configuration files as follows:

$ egrep "buffer_type|buffer_path" *.conf
output-es-config.conf:
  buffer_type file
  buffer_path `/var/lib/fluentd/buffer-output-es-config`
output-es-ops-config.conf:
  buffer_type file
  buffer_path `/var/lib/fluentd/buffer-output-es-ops-config`

The Fluentd buffer_queue_limit is the value of the variable BUFFER_QUEUE_LIMIT. This value is 32 by default.

The environment variable BUFFER_QUEUE_LIMIT is calculated as (FILE_BUFFER_LIMIT / (number_of_outputs * BUFFER_SIZE_LIMIT)).

If the BUFFER_QUEUE_LIMIT variable has the default set of values:

  • FILE_BUFFER_LIMIT = 256Mi

  • number_of_outputs = 1

  • BUFFER_SIZE_LIMIT = 8Mi

The value of buffer_queue_limit will be 32. To change the buffer_queue_limit, you must change the value of FILE_BUFFER_LIMIT.

In this formula, number_of_outputs is 1 if all the logs are sent to a single resource, and it is incremented by 1 for each additional resource. For example, the value of number_of_outputs is:

  • 1 - if all logs are sent to a single Elasticsearch pod

  • 2 - if application logs are sent to an Elasticsearch pod and ops logs are sent to another Elasticsearch pod

  • 4 - if application logs are sent to an Elasticsearch pod, ops logs are sent to another Elasticsearch pod, and both of them are forwarded to other Fluentd instances