Warning: This document is for the development version of Gnocchi. The main version is stable/4.1.

Running Gnocchi

Once Gnocchi is properly installed, you need to launch Gnocchi. Simply run the HTTP server and metric daemon:

gnocchi-api
gnocchi-metricd

You can run these services as background daemons.

Running API As A WSGI Application

To run Gnocchi API, you can use the provided gnocchi-api. It wraps around uwsgi – makes sure that uWSGI is installed. If one Gnocchi API server is not enough, you can spawn any number of new API server to scale Gnocchi out, even on different machines.

Since Gnocchi API tier runs using WSGI, it can alternatively be run using Apache httpd and mod_wsgi, or any other HTTP daemon.

uWSGI

If you want to deploy using uWSGI yourself, the following uWSGI configuration file can be used as a base:

[uwsgi]
http = localhost:8041
# Set the correct path depending on your installation
wsgi-file = /usr/local/bin/gnocchi-api
master = true
die-on-term = true
threads = 32
# Adjust based on the number of CPU
processes = 32
enabled-threads = true
thunder-lock = true
plugins = python
buffer-size = 65535
lazy-apps = true
add-header = Connection: close

You should configure the number of processes according to the number of CPU you have, usually around 1.5 × number of CPU.

Once written to /etc/gnocchi/uwsgi.ini, it can be launched this way:

uwsgi /etc/gnocchi/uwsgi.ini

Apache mod_wsgi

If you want to use Apache httpd mod_wsgi, here’s an example configuration file:

<VirtualHost *:8041>
  WSGIDaemonProcess gnocchi user=gnocchi processes=4 threads=32 display-name=%{GROUP}
  WSGIProcessGroup gnocchi
  WSGIScriptAlias / /usr/local/bin/gnocchi-api
  WSGIPassAuthorization On
  WSGIApplicationGroup %{GLOBAL}

  <Directory />
      Require all granted
  </Directory>
</VirtualHost>

How to define archive policies

The archive policies define how the metrics are aggregated and how long they are stored. Each archive policy definition is expressed as the number of points over a timespan.

If your archive policy defines a policy of 10 points with a granularity of 1 second, the time series archive will keep up to 10 seconds, each representing an aggregation over 1 second. This means the time series will at maximum retain 10 seconds of data (sometimes a bit more) between the more recent point and the oldest point. That does not mean it will be 10 consecutive seconds: there might be a gap if data is fed irregularly.

There is no expiry of data relative to the current timestamp. Data is only expired according to timespan.

Each archive policy also defines which aggregation methods will be used. The default is set to default_aggregation_methods which is by default set to mean, min, max, sum, std, count.

Therefore, both the archive policy and the granularity entirely depends on your use case. Depending on the usage of your data, you can define several archive policies. A typical low grained use case could be:

1440 points with a granularity of 1 minute = 24 hours

The worst case scenario for storing compressed data points is 8.04 bytes per point, whereas best case scenario can compress up to 0.05 bytes per point. Knowing that, it is possible to compute the worst case scenario for storage in order to plan for data storage capacity.

An archive policy of 1440 points would need 1440 points × 8.04 bytes = 11.3 KiB per aggregation method. If you use the 6 standard aggregation method proposed by Gnocchi, your metric will take up to 6 × 11.3 KiB = 67.8 KiB of disk space per metric.

Be aware that the more definitions you set in an archive policy, the more CPU it will consume. Therefore, creating an archive policy with 2 definitons (e.g. 1 second granularity for 1 day and 1 minute granularity for 1 month) may consume twice CPU than just one definition (e.g. just 1 second granularity for 1 day).

Default archive policies

By default, 4 archive policies are created when calling gnocchi-upgrade: bool, low, medium and high. The name both describes the storage space and CPU usage needs.

The bool archive policy is designed to store only boolean values (i.e. 0 and 1). It only stores one data point for each second (using the last aggregation method), with a one year retention period. The maximum optimistic storage size is estimated based on the assumption that no other value than 0 and 1 are sent as measures. If other values are sent, the maximum pessimistic storage size is taken into account.

  • low
    • 5 minutes granularity over 30 days
    • aggregation methods used: default_aggregation_methods
    • maximum estimated size per metric: 406 KiB
  • medium
    • 1 minute granularity over 7 days
    • 1 hour granularity over 365 days
    • aggregation methods used: default_aggregation_methods
    • maximum estimated size per metric: 887 KiB
  • high
    • 1 second granularity over 1 hour
    • 1 minute granularity over 1 week
    • 1 hour granularity over 1 year
    • aggregation methods used: default_aggregation_methods
    • maximum estimated size per metric: 1 057 KiB
  • bool
    • 1 second granularity over 1 year
    • aggregation methods used: last
    • maximum optimistic size per metric: 1 539 KiB
    • maximum pessimistic size per metric: 277 172 KiB

How to plan for Gnocchi’s storage

Gnocchi uses a custom file format based on its library Carbonara. In Gnocchi, a time series is a collection of points, where a point is a given aggregate or sample, in the lifespan of a time series. The storage format is compressed using various techniques, therefore the computing of a time series‘ size can be estimated based on its worst case scenario with the following formula:

number of points × 8 bytes = size in bytes

The number of points you want to keep is usually determined by the following formula:

number of points = timespan ÷ granularity

For example, if you want to keep a year of data with a one minute resolution:

number of points = (365 days × 24 hours × 60 minutes) ÷ 1 minute
number of points = 525 600

Then:

size in bytes = 525 600 points × 8 bytes = 4 204 800 bytes = 4 106 KiB

This is just for a single aggregated time series. If your archive policy uses the 6 default aggregation methods (mean, min, max, sum, std, count) with the same “one year, one minute aggregations” resolution, the space used will go up to a maximum of 6 × 4.1 MiB = 24.6 MiB.

Metricd

Metricd is the daemon responsible for processing measures, computing their aggregates and storing them into the aggregate storage. It also handles a few other cleanup tasks, such as deleting metrics marked for deletion.

Metricd therefore is responsible for most of the CPU usage and I/O job in Gnocchi. The archive policy of each metric will influence how fast it performs.

In order to process new measures, metricd checks the incoming storage for new measures from time to time. The delay between each check is can be configured by changing the [metricd]metric_processing_delay configuration option.

Some incoming driver (only Redis currently) are able to inform metricd that new measures are available for processing. In that case, metricd will not respect the [metricd]metric_processing_delay parameter and start processing the new measures right away. This behaviour can be disabled by turning off the [metricd]greedy option.

How many metricd workers do I need to run

By default, gnocchi-metricd daemon spans all your CPU power in order to maximize CPU utilisation when computing metric aggregation. You can use the gnocchi status command to query the HTTP API and get the cluster status for metric processing. It’ll show you the number of metric to process, known as the processing backlog for gnocchi-metricd. As long as this backlog is not continuously increasing, that means that gnocchi-metricd is able to cope with the amount of metric that are being sent. In case this number of measures to process is continuously increasing, you will need to (maybe temporarily) increase the number of gnocchi-metricd daemons. You can run any number of metricd daemon on any number of servers.

How to scale measure processing

Measurement data pushed to Gnocchi is divided into sacks for better distribution. The number of partitions is controlled by the sacks option under the [incoming] section. This value should be set based on the number of active metrics the system will capture. Additionally, the number of sacks, should be higher than the total number of active metricd workers. distribution. Incoming metrics are pushed to specific sacks and each sack is assigned to one or more gnocchi-metricd daemons for processing.

How many sacks do we need to create

This number of sacks enabled should be set based on the number of active metrics the system will capture. Additionally, the number of sacks, should be higher than the total number of active gnocchi-metricd workers.

In general, use the following equation to determine the appropriate sacks value to set:

sacks value = number of **active** metrics / 300

If the estimated number of metrics is the absolute maximum, divide the value by 500 instead. If the estimated number of active metrics is conservative and expected to grow, divide the value by 100 instead to accommodate growth.

How do we change sack size

In the event your system grows to capture signficantly more metrics than originally anticipated, the number of sacks can be changed to maintain good distribution. To avoid any loss of data when modifying sacks option. The option should be changed in the following order:

1. Stop all input services (api, statsd)

2. Stop all metricd services once backlog is cleared

3. Run gnocchi-change-sack-size <number of sacks> to set new sack size. Note
   that sack value can only be changed if the backlog is empty.

4. Restart all gnocchi services (api, statsd, metricd) with new configuration

Alternatively, to minimise API downtime:

1. Run gnocchi-upgrade but use a new incoming storage target such as a new
   ceph pool, file path, etc... Additionally, set |aggregate| storage to a
   new target as well.

2. Run gnocchi-change-sack-size <number of sacks> against new target

3. Stop all input services (api, statsd)

4. Restart all input services but target newly created incoming storage

5. When done clearing backlog from original incoming storage, switch all
   metricd datemons to target new incoming storage but maintain original
   |aggregate| storage.

How to monitor Gnocchi

The /v1/status endpoint of the HTTP API returns various information, such as the number of measures to process (measures backlog), which you can easily monitor (see How many metricd workers do I need to run). The Gnocchi client can show this output by running gnocchi status.

Making sure that the HTTP server and gnocchi-metricd daemon are running and are not writing anything alarming in their logs is a sign of good health of the overall system.

Total measures for backlog status may not accurately reflect the number of points to be processed when measures are submitted via batch.

How to backup and restore Gnocchi

In order to be able to recover from an unfortunate event, you need to backup both the index and the storage. That means creating a database dump (PostgreSQL or MySQL) and doing snapshots or copy of your data storage (Ceph, S3, Swift or your file system). The procedure to restore is no more complicated than initial deployment: restore your index and storage backups, reinstall Gnocchi if necessary, and restart it.

How to clear Gnocchi data

If you ever want to start fresh or need to clean Gnocchi data, this can be easily done. You need to clean the measures (incoming), aggregates (storage) and indexer data storage.

Once that is done, if you want to re-initialize Gnocchi, you need to call gnocchi-upgrade so it re-initialize the different drivers.

Index storage

Both MySQL and PostgreSQL drivers uses a single database. Delete the database. If you want to install Gnocchi again, create back that database with the same name before calling gnocchi-upgrade.

Incoming data

Depending on the driver you use, the data are stored in different places:

  • Ceph: delete the gnocchi-config object and the objects whose names start with incoming in the Ceph pool. Alternatively you can delete the Ceph pool (and recreate it if needed).
  • OpenStack Swift: delete the gnocchi-config container and containers whose names start with incoming in the Swift account.
  • Redis: delete the gnocchi-config key and the keys whose names start with incoming.
  • File: delete ${incoming.file_basepath}/tmp and the directories whose names start with ${incoming.file_basepath}/incoming.
  • Amazon S3: delete the bucket whose name start with incoming.

Storage data

Depending on the driver you use, the data are stored in different places:

  • Ceph: delete the objects whose names start with gnocchi_ in the Ceph pool. Alternatively you can delete the Ceph pool (and recreate it if needed).
  • OpenStack Swift: delete the containers whose names start with $storage.swift_container_prefix in the Swift account.
  • Redis: delete the keys whose names start with timeseries.
  • File: delete the directories whose names are UUIDs under $incoming.file_basepath.
  • Amazon S3: delete the bucket whose name start with $storage.s3_bucket_prefix.