106 lines
5.1 KiB
Markdown
106 lines
5.1 KiB
Markdown
---
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layout: "docs"
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page_title: "Server Performance"
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sidebar_current: "docs-guides-performance"
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description: |-
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Consul requires different amounts of compute resources, depending on cluster size and expected workload. This guide provides guidance on choosing compute resources.
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---
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# Server Performance
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Since Consul servers run a [consensus protocol](/docs/internals/consensus.html) to
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process all write operations and are contacted on nearly all read operations, server
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performance is critical for overall throughput and health of a Consul cluster. Servers
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are generally I/O bound for writes because the underlying Raft log store performs a sync
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to disk every time an entry is appended. Servers are generally CPU bound for reads since
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reads work from a fully in-memory data store that is optimized for concurrent access.
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<a name="minimum"></a>
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## Minimum Server Requirements
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In Consul 0.7, the default server [performance parameters](/docs/agent/options.html#performance)
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were tuned to allow Consul to run reliably (but relatively slowly) on a server cluster of three
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[AWS t2.micro](https://aws.amazon.com/ec2/instance-types/) instances. These thresholds
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were determined empirically using a leader instance that was under sufficient read, write,
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and network load to cause it to permanently be at zero CPU credits, forcing it to the baseline
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performance mode for that instance type. Real-world workloads typically have more bursts of
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activity, so this is a conservative and pessimistic tuning strategy.
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This default was chosen based on feedback from users, many of whom wanted a low cost way
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to run small production or development clusters with low cost compute resources, at the
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expense of some performance in leader failure detection and leader election times.
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The default performance configuration is equivalent to this:
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```javascript
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{
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"performance": {
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"raft_multiplier": 5
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}
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}
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```
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<a name="production"></a>
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## Production Server Requirements
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When running Consul 0.7 and later in production, it is recommended to configure the server
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[performance parameters](/docs/agent/options.html#performance) back to Consul's original
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high-performance settings. This will let Consul servers detect a failed leader and complete
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leader elections much more quickly than the default configuration which extends key Raft
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timeouts by a factor of 5, so it can be quite slow during these events.
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The high performance configuration is simple and looks like this:
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```javascript
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{
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"performance": {
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"raft_multiplier": 1
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}
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}
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```
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It's best to benchmark with a realistic workload when choosing a production server for Consul.
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Here are some general recommendations:
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* Consul will make use of multiple cores, and at least 2 cores are recommended.
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* For write-heavy workloads, disk speed on the servers is key for performance. Use SSDs or
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another fast disk technology for the best write throughput.
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* <a name="last-contact"></a>Spurious leader elections can be caused by networking issues between
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the servers or insufficient CPU resources. Users in cloud environments often bump their servers
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up to the next instance class with improved networking and CPU until leader elections stabilize,
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and in Consul 0.7 or later the [performance parameters](/docs/agent/options.html#performance)
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configuration now gives you tools to trade off performance instead of upsizing servers. You can
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use the [`consul.raft.leader.lastContact` telemetry](/docs/agent/telemetry.html#last-contact)
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to observe how the Raft timing is performing and guide the decision to de-tune Raft performance
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or add more powerful servers.
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* For DNS-heavy workloads, configuring all Consul agents in a cluster with the
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[`allow_stale`](/docs/agent/options.html#allow_stale) configuration option will allow reads to
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scale across all Consul servers, not just the leader. Consul 0.7 and later enables stale reads
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for DNS by default. See [Stale Reads](/docs/guides/dns-cache.html#stale) in the
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[DNS Caching](/docs/guides/dns-cache.html) guide for more details. It's also good to set
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reasonable, non-zero [DNS TTL values](/docs/guides/dns-cache.html#ttl) if your clients will
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respect them.
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* In other applications that perform high volumes of reads against Consul, consider using the
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[stale consistency mode](/api/index.html#consistency) available to allow reads to scale
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across all the servers and not just be forwarded to the leader.
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## Memory Requirements
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Consul server agents operate on a working set of data comprised of key/value
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entries, the service catalog, prepared queries, access control lists, and
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sessions in memory. These data are persisted through Raft to disk in the form
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of a snapshot and log of changes since the previous snapshot for durability.
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When planning for memory requirements, you should typically allocate
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enough RAM for your server agents to contain between 2 to 4 times the working
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set size. You can determine the working set size by noting the value of
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`consul.runtime.alloc_bytes` in the [Telemetry data](/docs/agent/telemetry.html).
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> NOTE: Consul is not designed to serve as a general purpose database, and you
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> should keep this in mind when choosing what data are populated to the
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> key/value store.
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