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Version: 21.11

Performance and storage tuning

This document contains guidelines for configuring your Redpanda cluster in the most optimal way to increase performance and reduce failures. We recommend that you consider the configurations below if you want to run Redpanda in a production environment.

Disk utilization

When a Redpanda node runs out of disk space, it terminates. This results in node failure and performance degradation. After a node runs out of disk space, you will not be able to prevent node failure by changing retention settings or deleting data unless a ballast file has been created.

If there are enough out-of-disk node failures in the cluster, performance can be greatly impacted and topics can become unavailable. For this reason, it’s important to configure retention and storage settings in a way that ensures the production stability of the cluster.

This document provides information on how to avoid out-of-disk failures by making the following configurations:

  • Retention
    • Time-based retention
    • Size-based retention
  • Segment size
  • Ballast file

Retention

Retention refers to the conditions under which messages in a topic are retained on disk. When retention conditions are exceeded, cleanup occurs. You can configure retention settings so that cleanup occurs under the following conditions:

  • Message age is exceeded
  • Aggregate message size in the topic is exceeded
  • Either message age is exceeded or aggregate message size in the topic is exceeded

The following table shows the parameters that are described in this section along with their scope and defaults. If no value is specified in the topic-level configuration, the topic uses the value of the host-wide configuration. Note that properties in redpanda.yaml use snake case while topic-level configurations use dots:

Type of retentionHost-wide configuration
(redpanda.yaml)
Topic-level configuration
(overrides host-wide configuration)
Time-baseddelete_retention_ms
Default - 604800000
retention.ms
No default
Size-basedretention_bytes
Default - null
retention.bytes
No default
Segment sizelog_segment_size
Default - 1073741824
N/A

Time-based retention

Configure time-based retention with the following properties:

  • delete_retention_ms
  • retention.ms

A message is deleted if its age exceeds the delete_retention_ms or retention.ms value. The parameters are explained in detail below:

  • delete_retention_ms - Host-wide setting in redpanda.yaml that specifies how long a message stays on disk before it is deleted. To set retention time for a single topic, use the retention.ms parameter, which overrides the delete_retention_ms setting. If retention.ms is not set at the topic-level, the topic uses the delete_retention_ms value.

    To minimize the chance of out-of-disk outages, you can set delete_retention_ms to 86400000, which is one day. Default is 604800000, which is one week.

  • retention.ms - Topic-level setting that specifies how long a message stays on disk before it is deleted. This setting overrides the host-wide delete_retention_ms parameter for the topic. To minimize the chance of out-of-disk outages, you can set retention.ms to 86400000, which is one day. There is no default, but if retention.ms is not set at the topic-level, the topic uses the delete_retention_ms value.

    You can set retention.ms with this command:

    rpk topic alter-config <topic> --set retention.ms=<retention_time>
caution

Do not set delete_retention_ms to -1 unless you are using remote write with Tiered Storage to upload segments to remote storage. Setting delete_retention_ms to -1 configures indefinite retention, which can result in an out-of-disk outage and may result in data loss.

Size-based retention

Configure size-based retention with the following properties:

  • retention_bytes
  • retention.bytes

A message is deleted if the size of the partition in which it is contained reaches the retention_bytes or retention.bytes value. The parameters are explained in detail below:

  • retention_bytes - Host-wide setting that specifies the maximum size of a partition. When a partition reaches the retention_bytes size, the oldest messages in the partition are deleted. To set retention size for a single topic, use the retention.bytes parameter, which overrides the retention_bytes setting. If retention.bytes is not set at the topic-level, the topic uses the retention_bytes value.

    We recommend that you configure retention_bytes to a value that is lower than the disk capacity, or a fraction of the disk capacity based on the number of partitions per topic. For example, if you have one partition, retention_bytes can be 80% of the disk size, or if you have 10 partitions, retention_byes can be 80% of the disk size divided by 10. Default is null, which means that retention based on topic size is disabled.

  • retention.bytes - Topic-level setting that specifies the maximum size of a partition. This setting overrides the host-wide retention_bytes parameter for the topic. We recommend the same considerations for the retention.bytes parameter as the retention_bytes parameter described above. There is no default, but if retention.bytes is not set at the topic-level, the topic uses the retention_bytes value.

    You can set retention.bytes with this command:

    rpk topic alter-config <topic> --set retention.bytes=<retention_size>

Segment size

The log_segment_size parameter specifies the size of each log segment.

You can set the log segment size globally in the redpanda.yaml file, but if you know which topics will receive more data, it is better to set the size for each topic. Configure log segment size on a topic with the following rpk command:

rpk topic alter-config <topic> --set segment.bytes=<segment_size>

When determining ideal segment size, keep in mind that very large segments prevent compaction and very small segments increase the risk of encountering resource limits. As a rule of thumb to calculate an upper limit on segment size, you can divide the disk size by the number of partitions. For example, if you have a 128GB disk and 1000 partitions, the upper limit of the segment size would be 134217728. Default is 1073741824.

Ballast file

You can enable ballast file creation to act as a buffer against an out-of-disk outage. The ballast file is an empty file that takes up disk space. In the event that Redpanda becomes unavailable because it runs out of disk space, you can delete the ballast file, which will clear up some disk space and give you time to delete topics or records and change your retention settings. Deletion of the ballast file is an emergency last resort and should not be considered a replacement for adjusting settings for segment size and retention.

To enable ballast file creation, configure the following parameters in the redpanda.yaml file:

tune_ballast_file: false
ballast_file_path: "/var/lib/redpanda/data/ballast"
ballast_file_size: "1GiB"

The parameters are explained in detail below:

  • tune_ballast_file - Tells Redpanda to create a ballast file. Set to true to enable ballast file creation. Default is false.

  • ballast_file_path - The location of the ballast file. You can change the location of the ballast file, but it must be on the same mount point as the Redpanda data directory. Default is /var/lib/redpanda/data/ballast.

  • ballast_file_size - The size of the ballast file. You can increase the ballast file size if it is a very high throughput cluster or decrease the ballast file size if you have very little storage space. You want to balance the size of the ballast file so that it is large enough to give you enough time to delete data and configure retention settings if Redpanda crashes, but small enough that you do not waste unnecessary disk space. In general, you can set ballast_file_size to approximately 10 times the size of the largest segment to have enough space to compact that topic. Default is 1GiB.

Cloud I/O optimization

Redpanda relies on its own disk I/O scheduler, and by default tells the kernel to use the noop scheduler. To give the users near-optimal performance by default, rpk comes with an embedded database of I/O settings for well-known cloud computers, which are specific combinations of CPUs, SSD types, and VM sizes. It is not the same to run software on four VCPUs than it is to run on an EC2 i3.metal with 96 physical cores. Often, when trying to scale rapidly to meet demands, product teams will not have the time to measure I/O throughput and latency before starting every new instance (via rpk iotune) and instead need resources right now. To meet this demand, Redpanda will attempt to predict the best known settings for VM cloud types.

We still encourage users to run rpk iotune for production workloads, but we’ll do our best to start with near optimal settings for popular machines types we have measured. It’s important to mention that this doesn’t need to be done each time Redpanda is started. rpk iotune’s output parameters are written to a file which can be saved and reused in nodes running on the same type of hardware.

Currently, we only have well-known-types for AWS and GCP (Azure VM types support is coming soon). Upon startup, rpk will try to detect the current cloud and instance type via the cloud’s metadata API, setting the correct iotune properties if the detected setup is known apriori.

If access to the metadata API isn’t allowed from the instance, you can also hint the desired setup by passing the --well-known-io flag to rpk start with the cloud vendor, VM type and storage type surrounded by quotes and separated by colons:

rpk start --well-known-io 'aws:i3.xlarge:default'

It can also be specified in the redpanda.yaml configuration file, under the rpk object:

rpk:
well_known_io: 'gcp:c2-standard-16:nvme'

If well-known-io is specified in the config file, and as a flag, the value passed with the flag will take precedence.

In the case where a certain cloud vendor, machine type or storage type isn’t found, or if the metadata isn’t available and no hint is given, rpk will print a warning pointing out the issue and continue using the default values.