Clarify disabling swap in docs
Our strong recommendation is disabling swap over any other alternative to avoid the JVM from landing on disk. This commit clarifies the docs in this regard.
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Jason Tedor
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@ -2,32 +2,64 @@
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=== Disable swapping
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Most operating systems try to use as much memory as possible for file system
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caches and eagerly swap out unused application memory. This can result in
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parts of the JVM heap being swapped out to disk.
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caches and eagerly swap out unused application memory. This can result in parts
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of the JVM heap or even its executable pages being swapped out to disk.
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Swapping is very bad for performance and for node stability and should be
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avoided at all costs. It can cause garbage collections to last for **minutes**
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instead of milliseconds and can cause nodes to respond slowly or even to
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disconnect from the cluster.
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Swapping is very bad for performance, for node stability, and should be avoided
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at all costs. It can cause garbage collections to last for **minutes** instead
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of milliseconds and can cause nodes to respond slowly or even to disconnect
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from the cluster. In a resilient distributed system, it's more effective to let
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the operating system kill the node.
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There are three approaches to disabling swapping:
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There are three approaches to disabling swapping. The preferred option is to
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completely disable swap. If this is not an option, whether or not to prefer
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minimizing swappiness versus memory locking is dependent on your environment.
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[[disable-swap-files]]
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==== Disable all swap files
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Usually Elasticsearch is the only service running on a box, and its memory usage
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is controlled by the JVM options. There should be no need to have swap enabled.
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On Linux systems, you can disable swap temporarily by running:
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[source,sh]
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--------------
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sudo swapoff -a
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--------------
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To disable it permanently, you will need to edit the `/etc/fstab` file and
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comment out any lines that contain the word `swap`.
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On Windows, the equivalent can be achieved by disabling the paging file entirely
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via `System Properties → Advanced → Performance → Advanced → Virtual memory`.
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[[swappiness]]
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==== Configure `swappiness`
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Another option available on Linux systems is to ensure that the sysctl value
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`vm.swappiness` is set to `1`. This reduces the kernel's tendency to swap and
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should not lead to swapping under normal circumstances, while still allowing the
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whole system to swap in emergency conditions.
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[[mlockall]]
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==== Enable `bootstrap.memory_lock`
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The first option is to use
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http://opengroup.org/onlinepubs/007908799/xsh/mlockall.html[mlockall] on Linux/Unix systems, or https://msdn.microsoft.com/en-us/library/windows/desktop/aa366895%28v=vs.85%29.aspx[VirtualLock] on Windows, to
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try to lock the process address space into RAM, preventing any Elasticsearch
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memory from being swapped out. This can be done, by adding this line
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to the `config/elasticsearch.yml` file:
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Another option is to use
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http://opengroup.org/onlinepubs/007908799/xsh/mlockall.html[mlockall] on
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Linux/Unix systems, or
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https://msdn.microsoft.com/en-us/library/windows/desktop/aa366895%28v=vs.85%29.aspx[VirtualLock]
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on Windows, to try to lock the process address space into RAM, preventing any
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Elasticsearch memory from being swapped out. This can be done, by adding this
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line to the `config/elasticsearch.yml` file:
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[source,yaml]
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--------------
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bootstrap.memory_lock: true
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--------------
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WARNING: `mlockall` might cause the JVM or shell session to exit if it tries
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to allocate more memory than is available!
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WARNING: `mlockall` might cause the JVM or shell session to exit if it tries to
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allocate more memory than is available!
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After starting Elasticsearch, you can see whether this setting was applied
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successfully by checking the value of `mlockall` in the output from this
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@ -40,11 +72,12 @@ GET _nodes?filter_path=**.mlockall
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// CONSOLE
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If you see that `mlockall` is `false`, then it means that the `mlockall`
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request has failed. You will also see a line with more information in the
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logs with the words `Unable to lock JVM Memory`.
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request has failed. You will also see a line with more information in the logs
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with the words `Unable to lock JVM Memory`.
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The most probable reason, on Linux/Unix systems, is that the user running
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Elasticsearch doesn't have permission to lock memory. This can be granted as follows:
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Elasticsearch doesn't have permission to lock memory. This can be granted as
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follows:
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`.zip` and `.tar.gz`::
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@ -55,13 +88,13 @@ Elasticsearch doesn't have permission to lock memory. This can be granted as fo
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RPM and Debian::
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Set `MAX_LOCKED_MEMORY` to `unlimited` in the
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<<sysconfig,system configuration file>> (or see below for systems using `systemd`).
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<<sysconfig,system configuration file>> (or see below for systems using
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`systemd`).
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Systems using `systemd`::
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Set `LimitMEMLOCK` to `infinity` in the <<systemd,systemd configuration>>.
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Another possible reason why `mlockall` can fail is that the temporary directory
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(usually `/tmp`) is mounted with the `noexec` option. This can be solved by
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specifying a new temp directory using the `ES_JAVA_OPTS` environment variable:
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@ -73,26 +106,3 @@ export ES_JAVA_OPTS="$ES_JAVA_OPTS -Djava.io.tmpdir=/path/to/temp/dir"
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--------------
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or setting this JVM flag in the jvm.options configuration file.
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[[disable-swap-files]]
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==== Disable all swap files
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The second option is to completely disable swap. Usually Elasticsearch
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is the only service running on a box, and its memory usage is controlled
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by the JVM options. There should be no need to have swap enabled.
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On Linux systems, you can disable swap temporarily
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by running: `sudo swapoff -a`. To disable it permanently, you will need
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to edit the `/etc/fstab` file and comment out any lines that contain the
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word `swap`.
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On Windows, the equivalent can be achieved by disabling the paging file entirely
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via `System Properties → Advanced → Performance → Advanced → Virtual memory`.
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[[swappiness]]
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==== Configure `swappiness`
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Another option available on Linux systems is to ensure that the sysctl value
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`vm.swappiness` is set to `1`. This reduces the kernel's tendency to swap and
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should not lead to swapping under normal circumstances, while still allowing
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the whole system to swap in emergency conditions.
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