VictoriaMetrics/docs/FAQ.md

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# FAQ

## What is the main purpose of VictoriaMetrics?

To provide the best monitoring solution.

## Who uses VictoriaMetrics?

See [case studies](https://docs.victoriametrics.com/CaseStudies.html).

## Which features does VictoriaMetrics have?

See [these docs](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#prominent-features).

## Are there performance comparisons with other solutions?

Yes. See [these benchmarks](https://docs.victoriametrics.com/Articles.html#benchmarks).

## How to start using VictoriaMetrics?

See [these docs](https://docs.victoriametrics.com/Quick-Start.html).

## Does VictoriaMetrics support replication?

Yes. See [these docs](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#replication-and-data-safety) for details.

## Can I use VictoriaMetrics instead of Prometheus?

Yes in most cases. VictoriaMetrics can substitute Prometheus in the following aspects:

* Prometheus-compatible service discovery and target scraping can be done with [vmagent](https://docs.victoriametrics.com/vmagent.html) and with single-node VictoriaMetrics. See [these docs](https://docs.victoriametrics.com/#how-to-scrape-prometheus-exporters-such-as-node-exporter).
* Prometheus-compatible alerting rules and recording rules can be processed with [vmalert](https://docs.victoriametrics.com/vmalert.html).
* Prometheus-compatible querying in Grafana is supported by VictoriaMetrics. See [these docs](https://docs.victoriametrics.com/#grafana-setup).

## What is the difference between vmagent and Prometheus?

While both [vmagent](https://docs.victoriametrics.com/vmagent.html) and Prometheus may scrape Prometheus targets (aka `/metrics` pages)
according to the provided Prometheus-compatible [scrape configs](https://docs.victoriametrics.com/sd_configs.html#scrape_configs)
and send data to multiple remote storage systems, vmagent has the following additional features:

* vmagent usually requires lower amounts of CPU, RAM and disk IO compared to Prometheus when scraping an enormous number of targets (more than 1000)
  or targets with a great number of exposed metrics.
* vmagent provides independent disk-backed buffers for each configured remote storage (see `-remoteWrite.url`). This means that slow or temporarily unavailable storage
  doesn't prevent it from sending data to healthy storage in parallel. Prometheus uses a single shared buffer for all the configured remote storage systems (see `remote_write->url`)
  with a hardcoded retention of 2 hours.
* vmagent may accept, relabel and filter data obtained via multiple data ingestion protocols in addition to data scraped from Prometheus targets.
  That means it supports both `pull` and `push` protocols for data ingestion.
  See [these docs](https://docs.victoriametrics.com/vmagent.html#features) for details.
* vmagent may be used in different use cases:
  * [IoT and edge monitoring](https://docs.victoriametrics.com/vmagent.html#iot-and-edge-monitoring)
  * [Drop-in replacement for Prometheus](https://docs.victoriametrics.com/vmagent.html#drop-in-replacement-for-prometheus)
  * [Replication and High Availability](https://docs.victoriametrics.com/vmagent.html#replication-and-high-availability)
  * [Relabeling and Filtering](https://docs.victoriametrics.com/vmagent.html#relabeling-and-filtering)
  * [Splitting data streams among multiple systems](https://docs.victoriametrics.com/vmagent.html#splitting-data-streams-among-multiple-systems)
  * [Prometheus remote_write proxy](https://docs.victoriametrics.com/vmagent.html#prometheus-remote_write-proxy)

## What is the difference between vmagent and Prometheus agent?

Both [vmagent](https://docs.victoriametrics.com/vmagent.html) and [Prometheus agent](https://prometheus.io/blog/2021/11/16/agent/) serve the same purpose – to efficently scrape Prometheus-compatible targets at the edge. They have the following differences:

* vmagent usually requires lower amounts of CPU, RAM and disk IO compared to the Prometheus agent.
* Prometheus agent supports only pull-based data collection (e.g. it can scrape Prometheus-compatible targets), while vmagent supports both pull and push data collection – it can accept data via many popular data ingestion protocols such as InfluxDB line protocol, Graphite protocol, OpenTSDB protocol, DataDog protocol, Prometheus protocol, CSV and JSON – see [these docs](https://docs.victoriametrics.com/vmagent.html#features).
* vmagent can easily scale horizontally to multiple instances for scraping a big number of targets – see [these docs](https://docs.victoriametrics.com/vmagent.html#scraping-big-number-of-targets).
* vmagent supports [improved relabeling](https://docs.victoriametrics.com/vmagent.html#relabeling).
* vmagent can limit the number of scraped metrics per target – see [these docs](https://docs.victoriametrics.com/vmagent.html#cardinality-limiter).
* vmagent supports loading scrape configs from multiple files – see [these docs](https://docs.victoriametrics.com/vmagent.html#loading-scrape-configs-from-multiple-files).
* vmagent supports data reading and data writing from/to Kafka – see [these docs](https://docs.victoriametrics.com/vmagent.html#kafka-integration).
* vmagent can read and update scrape configs from http and https URLs, while the Prometheus agent can read them only from the local file system.

## Is it safe to enable [remote write](https://prometheus.io/docs/operating/integrations/#remote-endpoints-and-storage) in Prometheus?

Yes. Prometheus continues writing data to local storage after enabling remote write, so all the existing local storage data
and new data is available for querying via Prometheus as usual.

It is recommended using [vmagent](https://docs.victoriametrics.com/vmagent.html) for scraping Prometheus targets
and writing data to VictoriaMetrics.

## How does VictoriaMetrics compare to other remote storage solutions for Prometheus such as [M3 from Uber](https://eng.uber.com/m3/), [Thanos](https://github.com/thanos-io/thanos), [Cortex](https://github.com/cortexproject/cortex), etc.?

VictoriaMetrics is simpler, faster, more cost-effective and it provides [MetricsQL query language](MetricsQL) based on PromQL. The simplicity is twofold:
* It is simpler to configure and operate. There is no need for configuring [sidecars](https://github.com/thanos-io/thanos/blob/master/docs/components/sidecar.md),
  fighting the [gossip protocol](https://github.com/improbable-eng/thanos/blob/030bc345c12c446962225221795f4973848caab5/docs/proposals/completed/201809_gossip-removal.md)
  or setting up third-party systems such as [Consul](https://github.com/cortexproject/cortex/issues/157), [Cassandra](https://cortexmetrics.io/docs/chunks-storage/running-chunks-storage-with-cassandra/),
  [DynamoDB](https://cortexmetrics.io/docs/chunks-storage/aws-tips/) or [Memcached](https://cortexmetrics.io/docs/chunks-storage/caching/).
* VictoriaMetrics has a simpler architecture. This means fewer bugs and more useful features in the long run compared to competing TSDBs.

See [comparing Thanos to VictoriaMetrics cluster](https://medium.com/@valyala/comparing-thanos-to-victoriametrics-cluster-b193bea1683)
and the [Remote Write Storage Wars](https://promcon.io/2019-munich/talks/remote-write-storage-wars/) talk from [PromCon 2019](https://promcon.io/2019-munich/talks/remote-write-storage-wars/).

VictoriaMetrics also [uses less RAM than Thanos components](https://github.com/thanos-io/thanos/issues/448).

## What is the difference between VictoriaMetrics and [QuestDB](https://questdb.io/)?

* QuestDB needs more than 20x storage space than VictoriaMetrics. This translates to higher storage costs and slower queries over historical data, which must be read from the disk.
* QuestDB is much harder to set up and operate than VictoriaMetrics. Compare [setup instructions for QuestDB](https://questdb.io/docs/get-started/binaries) to [setup instructions for VictoriaMetrics](https://docs.victoriametrics.com/#how-to-start-victoriametrics).
* VictoriaMetrics provides the [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html) query language, which is better suited for typical queries over time series data than the SQL-like query language provided by QuestDB. See [this article](https://valyala.medium.com/promql-tutorial-for-beginners-9ab455142085) for details.
* VictoriaMetrics can be queried via the [Prometheus querying API](https://docs.victoriametrics.com/#prometheus-querying-api-usage) and via [Graphite's API](https://docs.victoriametrics.com/#graphite-api-usage).
* Thanks to PromQL support, VictoriaMetrics [can be used as a drop-in replacement for Prometheus in Grafana](https://docs.victoriametrics.com/#grafana-setup), while QuestDB needs a full rewrite of existing dashboards in Grafana.
* Thanks to Prometheus' remote_write API support, VictoriaMetrics can be used as a long-term storage for Prometheus or for [vmagent](https://docs.victoriametrics.com/vmagent.html), while QuestDB has no integration with Prometheus.
* QuestDB [supports a smaller range of popular data ingestion protocols](https://questdb.io/docs/develop/insert-data) compared to VictoriaMetrics (compare to [the list of supported data ingestion protocols for VictoriaMetrics](https://docs.victoriametrics.com/#how-to-import-time-series-data)).
* [VictoriaMetrics supports backfilling (e.g. storing historical data) out of the box](https://docs.victoriametrics.com/#backfilling), while QuestDB provides [very limited support for backfilling](https://questdb.io/blog/2021/05/10/questdb-release-6-0-tsbs-benchmark#the-problem-with-out-of-order-data).

## What is the difference between VictoriaMetrics and [Grafana Mimir](https://github.com/grafana/mimir)?

Grafana Mimir is a [Cortex](https://github.com/cortexproject/cortex) fork, so it has the same differences
as Cortex. See [what is the difference between VictoriaMetrics and Cortex](#what-is-the-difference-between-victoriametrics-and-cortex).

## What is the difference between VictoriaMetrics and [Cortex](https://github.com/cortexproject/cortex)?

VictoriaMetrics is similar to Cortex in the following aspects:

* Both systems accept data from [vmagent](https://docs.victoriametrics.com/vmagent.html) or Prometheus
  via the standard [remote_write API](https://prometheus.io/docs/practices/remote_write/), so there is no need for running sidecars
  unlike in [Thanos](https://github.com/thanos-io/thanos)' case.
* Both systems support multi-tenancy out of the box. See [the corresponding docs for VictoriaMetrics](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#multitenancy).
* Both systems support data replication. See [replication in Cortex](https://github.com/cortexproject/cortex/blob/fe56f1420099aa1bf1ce09316c186e05bddee879/docs/architecture.md#hashing) and [replication in VictoriaMetrics](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#replication-and-data-safety).
* Both systems scale horizontally to multiple nodes. See [these docs](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#cluster-resizing-and-scalability) for details.
* Both systems support alerting and recording rules via the corresponding tools such as [vmalert](https://docs.victoriametrics.com/vmalert.html).
* Both systems can be queried via the [Prometheus querying API](https://prometheus.io/docs/prometheus/latest/querying/api/) and integrate perfectly with Grafana.

The main differences between Cortex and VictoriaMetrics:

* Cortex re-uses Prometheus source code, while VictoriaMetrics is written from scratch.
* Cortex heavily relies on third-party services such as Consul, Memcache, DynamoDB, BigTable, Cassandra, etc.
  This may increase operational complexity and reduce system reliability compared to VictoriaMetrics' case,
  which doesn't use any external services. Compare [Cortex' Architecture](https://github.com/cortexproject/cortex/blob/master/docs/architecture.md)
  to [VictoriaMetrics' architecture](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#architecture-overview).
* VictoriaMetrics provides [production-ready single-node solution](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html),
  which is much easier to set up and operate than a Cortex cluster.
* Cortex may lose up to 12 hours of recent data on Ingestor failure – see [the corresponding docs](https://github.com/cortexproject/cortex/blob/fe56f1420099aa1bf1ce09316c186e05bddee879/docs/architecture.md#ingesters-failure-and-data-loss).
  VictoriaMetrics may lose only a few seconds of recent data, which isn't synced to persistent storage yet.
  See [this article for details](https://medium.com/@valyala/wal-usage-looks-broken-in-modern-time-series-databases-b62a627ab704).
* Cortex is usually slower and requires more CPU and RAM than VictoriaMetrics. See [this talk from adidas at PromCon 2019](https://promcon.io/2019-munich/talks/remote-write-storage-wars/) and [other case studies](https://docs.victoriametrics.com/CaseStudies.html).
* VictoriaMetrics accepts data in multiple popular data ingestion protocols additionally to Prometheus remote_write protocol – InfluxDB, OpenTSDB, Graphite, CSV, JSON, native binary.
  See [these docs](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#how-to-import-time-series-data) for details.
* VictoriaMetrics provides the [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html) query language, while Cortex provides the [PromQL](https://prometheus.io/docs/prometheus/latest/querying/basics/) query language.
* VictoriaMetrics can be queried via [Graphite's API](https://docs.victoriametrics.com/#graphite-api-usage).

## What is the difference between VictoriaMetrics and [Thanos](https://github.com/thanos-io/thanos)?

* Thanos re-uses Prometheus source code, while VictoriaMetrics is written from scratch.
* VictoriaMetrics accepts data via the [standard remote_write API for Prometheus](https://prometheus.io/docs/practices/remote_write/),
  while Thanos uses a non-standard [sidecar](https://github.com/thanos-io/thanos/blob/master/docs/components/sidecar.md) which must run alongside each Prometheus instance.
* The Thanos sidecar requires disabling data compaction in Prometheus, which may hurt Prometheus performance and increase RAM usage. See [these docs](https://thanos.io/tip/components/sidecar.md/) for more details.
* Thanos stores data in object storage (Amazon S3 or Google GCS), while VictoriaMetrics stores data in block storage
  ([GCP persistent disks](https://cloud.google.com/compute/docs/disks#pdspecs), Amazon EBS or bare metal HDD).
  While object storage is usually less expensive, block storage provides much lower latencies and higher throughput.
  VictoriaMetrics works perfectly with HDD-based block storage – there is no need for using more expensive SSD or NVMe disks in most cases.
* Thanos may lose up to 2 hours of recent data, which wasn't uploaded yet to object storage. VictoriaMetrics may lose only a few seconds of recent data,
  which hasn't been synced to persistent storage yet. See [this article for details](https://medium.com/@valyala/wal-usage-looks-broken-in-modern-time-series-databases-b62a627ab704).
* VictoriaMetrics provides a [production-ready single-node solution](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html),
  which is much easier to set up and operate than Thanos components.
* Thanos may be harder to set up and operate compared to VictoriaMetrics, since it has more moving parts, which can be connected with fewer reliable networks.
  See [this article for details](https://medium.com/faun/comparing-thanos-to-victoriametrics-cluster-b193bea1683).
* Thanos is usually slower and requires more CPU and RAM than VictoriaMetrics. See [this talk from adidas at PromCon 2019](https://promcon.io/2019-munich/talks/remote-write-storage-wars/).
* VictoriaMetrics accepts data via multiple popular data ingestion protocols in addition to the Prometheus remote_write protocol – InfluxDB, OpenTSDB, Graphite, CSV, JSON, native binary.
  See [these docs](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#how-to-import-time-series-data) for details.
* VictoriaMetrics provides the [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html) query language, while Thanos provides the [PromQL](https://prometheus.io/docs/prometheus/latest/querying/basics/) query language.
* VictoriaMetrics can be queried via [Graphite's API](https://docs.victoriametrics.com/#graphite-api-usage).

## How does VictoriaMetrics compare to [InfluxDB](https://www.influxdata.com/time-series-platform/influxdb/)?

* VictoriaMetrics requires [10x less RAM](https://medium.com/@valyala/insert-benchmarks-with-inch-influxdb-vs-victoriametrics-e31a41ae2893) and it [works faster](https://medium.com/@valyala/measuring-vertical-scalability-for-time-series-databases-in-google-cloud-92550d78d8ae).
* VictoriaMetrics needs lower amounts of storage space than InfluxDB for production data.
* VictoriaMetrics doesn't support InfluxQL or Flux but provides a better query language – [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html). See [this tutorial](https://medium.com/@valyala/promql-tutorial-for-beginners-9ab455142085) for details.
* VictoriaMetrics accepts data in multiple popular data ingestion protocols in addition to InfluxDB – Prometheus remote_write, OpenTSDB, Graphite, CSV, JSON, native binary.
  See [these docs](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#how-to-import-time-series-data) for details.
* VictoriaMetrics can be queried via [Graphite's API](https://docs.victoriametrics.com/#graphite-api-usage).

## How does VictoriaMetrics compare to [TimescaleDB](https://www.timescale.com/)?

* TimescaleDB insists on using SQL as a query language. While SQL is more powerful than PromQL, this power is rarely required during typical usages of a TSDB. Real-world queries usually [look clearer and simpler when written in PromQL than in SQL](https://medium.com/@valyala/promql-tutorial-for-beginners-9ab455142085).
* VictoriaMetrics requires [up to 70x less storage space compared to TimescaleDB](https://medium.com/@valyala/when-size-matters-benchmarking-victoriametrics-vs-timescale-and-influxdb-6035811952d4) for storing the same amount of time series data. The gap in storage space usage can be lowered from 70x to 3x if [compression in TimescaleDB is properly configured](https://docs.timescale.com/latest/using-timescaledb/compression) (it isn't an easy task in general :)).
* VictoriaMetrics requires up to 10x less CPU and RAM resources than TimescaleDB for processing production data. See [this article](https://abiosgaming.com/press/high-cardinality-aggregations/) for details.
* TimescaleDB is [harder to set up, configure and operate](https://docs.timescale.com/timescaledb/latest/how-to-guides/install-timescaledb/self-hosted/ubuntu/installation-apt-ubuntu/) than VictoriaMetrics (see [how to run VictoriaMetrics](https://docs.victoriametrics.com/#how-to-start-victoriametrics)).
* VictoriaMetrics accepts data in multiple popular data ingestion protocols – InfluxDB, OpenTSDB, Graphite, CSV – while TimescaleDB supports only SQL inserts.
* VictoriaMetrics can be queried via [Graphite's API](https://docs.victoriametrics.com/#graphite-api-usage).

## Does VictoriaMetrics use Prometheus technologies like other clustered TSDBs built on top of Prometheus such as [Thanos](https://github.com/thanos-io/thanos) or [Cortex](https://github.com/cortexproject/cortex)?

No. VictoriaMetrics core is written in Go from scratch by [fasthttp](https://github.com/valyala/fasthttp)'s [author](https://github.com/valyala).
The architecture is [optimized for storing and querying large amounts of time series data with high cardinality](https://medium.com/devopslinks/victoriametrics-creating-the-best-remote-storage-for-prometheus-5d92d66787ac). VictoriaMetrics storage uses [certain ideas from ClickHouse](https://medium.com/@valyala/how-victoriametrics-makes-instant-snapshots-for-multi-terabyte-time-series-data-e1f3fb0e0282). Special thanks to [Alexey Milovidov](https://github.com/alexey-milovidov).

## What is the pricing for VictoriaMetrics?

The following versions are open source and free:

* [Single-node version](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html).
* [Cluster version](https://github.com/VictoriaMetrics/VictoriaMetrics/tree/cluster).

We provide commercial support for both versions. [Contact us](mailto:info@victoriametrics.com) for the pricing.

The following commercial versions of VictoriaMetrics are available:

* [Managed VictoriaMetrics at AWS](https://aws.amazon.com/marketplace/pp/prodview-4tbfq5icmbmyc) (aka managed Prometheus).

The following commercial versions of VictoriaMetrics are planned:

* Managed VictoriaMetrics at Google Cloud.
* Cloud monitoring solution based on VictoriaMetrics.

[Contact us](mailto:info@victoriametrics.com) for more information on our plans.

## Why doesn't VictoriaMetrics support the [Prometheus remote read API](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#%3Cremote_read%3E)?

The remote read API requires transferring all the raw data for all the requested metrics over the given time range. For instance,
if a query covers 1000 metrics with 10K values each, then the remote read API has to return `1000*10K`=10M metric values to Prometheus.
This is slow and expensive.
Prometheus' remote read API isn't intended for querying foreign data – aka `global query view`. See [this issue](https://github.com/prometheus/prometheus/issues/4456) for details.

So just query VictoriaMetrics directly via [vmui](https://docs.victoriametrics.com/#vmui), the [Prometheus Querying API](https://docs.victoriametrics.com/#prometheus-querying-api-usage)
or via [Prometheus datasource in Grafana](https://docs.victoriametrics.com/#grafana-setup).

## Does VictoriaMetrics deduplicate data from Prometheus instances scraping the same targets (aka `HA pairs`)?

Yes. See [these docs](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#deduplication) for details.

## Where is the source code of VictoriaMetrics?

Source code for the following versions is available in the following places:

* [Single-node version](https://github.com/VictoriaMetrics/VictoriaMetrics)
* [Cluster version](https://github.com/VictoriaMetrics/VictoriaMetrics/tree/cluster)

## Is VictoriaMetrics a good fit for data from IoT sensors and industrial sensors?

VictoriaMetrics is able to handle data from hundreds of millions of IoT sensors and industrial sensors.
It supports [high cardinality data](https://medium.com/@valyala/high-cardinality-tsdb-benchmarks-victoriametrics-vs-timescaledb-vs-influxdb-13e6ee64dd6b),
perfectly [scales up on a single node](https://medium.com/@valyala/measuring-vertical-scalability-for-time-series-databases-in-google-cloud-92550d78d8ae)
and scales horizontally to multiple nodes.

## Where can I ask questions about VictoriaMetrics?

Questions about VictoriaMetrics can be asked via the following channels:

* [Slack channel](https://slack.victoriametrics.com/)
* [Telegram channel](https://t.me/VictoriaMetrics_en)
* [Google group](https://groups.google.com/forum/#!forum/victorametrics-users)

## Where can I file bugs and feature requests regarding VictoriaMetrics?

File bugs and feature requests [here](https://github.com/VictoriaMetrics/VictoriaMetrics/issues).

## Where can I find information about multi-tenancy?

See [these docs](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#multitenancy). Multitenancy is supported only by the [cluster version](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html) of VictoriaMetrics.

## How to set a memory limit for VictoriaMetrics components?

All the VictoriaMetrics components provide command-line flags to control the size of internal buffers and caches: `-memory.allowedPercent` and `-memory.allowedBytes` (pass `-help` to any VictoriaMetrics component in order to see the description for these flags). These limits don't take into account additional memory, which may be needed for processing incoming queries. Hard limits may be enforced only by the OS via [cgroups](https://en.wikipedia.org/wiki/Cgroups), Docker (see [these docs](https://docs.docker.com/config/containers/resource_constraints)) or Kubernetes (see [these docs](https://kubernetes.io/docs/concepts/configuration/manage-resources-containers)).

Memory usage for VictoriaMetrics components can be tuned according to the following docs:

* [Resource usage limits for single-node VictoriaMetrics](https://docs.victoriametrics.com/#resource-usage-limits)
* [Resource usage limits for cluster VictoriaMetrics](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#resource-usage-limits)
* [Troubleshooting for vmagent](https://docs.victoriametrics.com/vmagent.html#troubleshooting)
* [Troubleshooting for single-node VictoriaMetrics](https://docs.victoriametrics.com/#troubleshooting)

## How can I run VictoriaMetrics on FreeBSD/OpenBSD?

VictoriaMetrics is included in [OpenBSD](https://github.com/openbsd/ports/blob/c1bfea520bbb30d6e5f8d0f09115ace341f820d6/infrastructure/db/user.list#L383) and [FreeBSD](https://www.freebsd.org/cgi/ports.cgi?query=victoria&stype=all) ports so just install it from there or use pre-built binaries from [releases page](https://github.com/VictoriaMetrics/VictoriaMetrics/releases).

## Does VictoriaMetrics support the Graphite query language?

Yes. See [these docs](https://docs.victoriametrics.com/#graphite-api-usage).

## What is an active time series?

A time series is uniquely identified by its name plus a set of its labels. For example, `temperature{city="NY",country="US"}` and `temperature{city="SF",country="US"}` are two distinct series, since they differ by the `city` label. A time series is considered active if it receives at least a single new sample during the last hour.

## What is high churn rate?

If old time series are constantly substituted by new time series at a high rate, then such a state is called `high churn rate`. High churn rate has the following negative consequences:

* Increased total number of time series stored in the database.
* Increased size of inverted index, which is stored at `<-storageDataPath>/indexdb`, since the inverted index contains entries for every label of every time series with at least a single ingested sample.
* Slow-down of queries over multiple days.

The main reason for high churn rate is a metric label with frequently changed value. Examples of such labels:

* `queryid`, which changes with each query at `postgres_exporter`.
* `app_name` or `deployment_id`, which changes with each new deployment in Kubernetes.
* A label derived from the current time such as `timestamp`, `minute` or `hour`.
* A `hash` or `uuid` label, which changes frequently.

The solution against high churn rate is to identify and eliminate labels with frequently changed values. [Cardinality explorer](https://docs.victoriametrics.com/#cardinality-explorer) can help determining these labels.

## What is high cardinality?

High cardinality usually means a high number of [active time series](#what-is-an-active-time-series). High cardinality may lead to high memory usage and/or to a high percentage of [slow inserts](#what-is-a-slow-insert). The source of high cardinality is usually a label with a large number of unique values, which presents a big share of the ingested time series. The solution is to identify and remove the source of high cardinality with the help of [cardinality explorer](https://docs.victoriametrics.com/#cardinality-explorer).

## What is a slow insert?

VictoriaMetrics maintains in-memory cache for mapping of [active time series](#what-is-an-active-time-series) into internal series ids. The cache size depends on the available memory for VictoriaMetrics in the host system. If the information about all the active time series doesn't fit the cache, then VictoriaMetrics needs to read and unpack the information from disk on every incoming sample for time series missing in the cache. This operation is much slower than the cache lookup, so such an insert is named a `slow insert`. A high percentage of slow inserts on the [official dashboard for VictoriaMetrics](https://docs.victoriametrics.com/#monitoring) indicates a memory shortage for the current number of [active time series](#what-is-an-active-time-series). Such a condition usually leads to a significant slowdown for data ingestion and to significantly increased disk IO and CPU usage. The solution is to add more memory or to reduce the number of [active time series](#what-is-an-active-time-series). [Cardinality explorer](https://docs.victoriametrics.com/#cardinality-explorer) can be helpful for locating the source of high number of active time series.

## How to optimize MetricsQL query?

See [this article](https://valyala.medium.com/how-to-optimize-promql-and-metricsql-queries-85a1b75bf986).

## Why isn't MetricsQL 100% compatible with PromQL?

[MetricsQL](https://docs.victoriametrics.com/MetricsQL.html) provides better user experience than PromQL. It fixes a few annoying issues in PromQL. This prevents MetricsQL to be 100% compatible with PromQL. See [this article](https://medium.com/@romanhavronenko/victoriametrics-promql-compliance-d4318203f51e) for details.

## How to migrate data from Prometheus to VictoriaMetrics?

Please see [these docs](https://docs.victoriametrics.com/vmctl.html#migrating-data-from-prometheus).

## How to migrate data from InfluxDB to VictoriaMetrics?

Please see [these docs](https://docs.victoriametrics.com/vmctl.html#migrating-data-from-influxdb-1x).

## How to migrate data from OpenTSDB to VictoriaMetrics?

Please see [these docs](https://docs.victoriametrics.com/vmctl.html#migrating-data-from-opentsdb).

## How to migrate data from Graphite to VictoriaMetrics?

Please use the [whisper-to-graphite](https://github.com/bzed/whisper-to-graphite) tool for reading data from Graphite and pushing them to VictoriaMetrics via [Graphite's import API](https://docs.victoriametrics.com/#how-to-send-data-from-graphite-compatible-agents-such-as-statsd).

## Why do the same metrics have differences in VictoriaMetrics' and Prometheus' dashboards?

There could be a slight difference in stored values for time series. Due to different compression algorithms, VM may reduce the precision for float values with more than 12 significant decimal digits. Please see [this article](https://valyala.medium.com/evaluating-performance-and-correctness-victoriametrics-response-e27315627e87).

The query engine may behave differently for some functions. Please see [this article](https://medium.com/@romanhavronenko/victoriametrics-promql-compliance-d4318203f51e).

## If downsampling and deduplication are enabled how will this work?

[Deduplication](https://docs.victoriametrics.com/#deduplication) is a special case of zero-offset [downsampling](https://docs.victoriametrics.com/#downsampling). So, if both downsampling and deduplication are enabled, then deduplication is replaced by zero-offset downsampling

## How to upgrade or downgrade VictoriaMetrics without downtime?

Single-node VictoriaMetrics cannot be restarted / upgraded or downgraded without downtime, since it needs to be gracefully shut down and then started again. See [how to upgrade VictoriaMetrics](https://docs.victoriametrics.com/#how-to-upgrade-victoriametrics).

[Cluster version of VictoriaMetrics](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html) can be restarted / upgraded / downgraded without downtime if the following conditions are met:

* If every component of the cluster - `vminsert`, `vmselect` and `vmstorage` - has at least 2 instances.
* If the cluster has enough compute resources (CPU, RAM, disk IO, network bandwidth) to perform rolling restart of all its components.
* If the version used for upgrade / downgrade is compatible with the currently running version. The [CHANGELOG](https://docs.victoriametrics.com/CHANGELOG.html) contains compatibility notes for the published releases.

See [updating / reconfiguring cluster nodes](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#updating--reconfiguring-cluster-nodes) for details on cluster upgrade.