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What it means
File descriptors are required so that the Elasticsearch process can keep track of all the files it has open at any given time as well as all network connections to other nodes.
Running out of file descriptors would result in the Elasticsearch process not being able to keep track of the files it has open or not being able to open new files or socket connections when it needs to, and will most probably lead to data loss.
The Elasticsearch process should be permitted up to 65,536 or more.
How to resolve it
If you used systemd or yum to install Elasticsearch, then the setting should be applied automatically. If you installed a tarball then you will need to do the following:
Add the line:
elasticsearch - nofile 65535
MacOS users must also add the following line to the jvm.options file (not necessary for Linux):
You can check the setting has taken effect: