LimitMEMLOCK disappeared in Elasticsearch 5.6 deb package

[emmanuelGuiton]

Elasticsearch version (bin/elasticsearch --version): 5.6

Plugins installed: none

JVM version (java -version): 1.8.0_144

OS version (uname -a if on a Unix-like system): Debian 4.9.30-2+deb9u3

Description of the problem including expected versus actual behavior:
When upgrading from Elasticsearch 5.5.x to 5.6 using the deb repository, I noticed that limitMEMLOCK setting had disappeared from /usr/lib/systemd/system/elasticsearch.service while LimitAS and LimitFSIZE had appeared. I though the latter replaced the former. But when I started elasticsearch, there was an error :

[2017-09-14T10:11:33,055][ERROR][o.e.b.Bootstrap ] [ISC-LAB-ES011] node validation exception
[1] bootstrap checks failed
[1]: memory locking requested for elasticsearch process but memory is not locked
[2017-09-14T10:11:33,062][INFO ][o.e.n.Node ] [ISC-LAB-ES011] stopping ...
[2017-09-14T10:11:33,133][INFO ][o.e.n.Node ] [ISC-LAB-ES011] stopped
[2017-09-14T10:11:33,133][INFO ][o.e.n.Node ] [ISC-LAB-ES011] closing ...
[2017-09-14T10:11:33,151][INFO ][o.e.n.Node ] [ISC-LAB-ES011] closed

To fix it, I simply added back the LimitMEMLOCK setting.

Steps to reproduce:

1. Install Elasticsearch 5.6 from deb repository
2. Start it.

Provide logs (if relevant):

[jasontedor]

It was only ever commented out but we removed it. See #25979 for context. You should add a unit file instead of editing the service file which is one reason that we removed it.

[sergii-sakharov]

For those like me who don't know what a "unit file" is, exact file is mentioned in here:

https://www.elastic.co/guide/en/elasticsearch/reference/5.6/setting-system-settings.html#systemd

[jasontedor]

Thanks for linking to that @sergii-sakharov, I should have done that in the first place. I did edit your link inline from current to 5.6 because current changes with time and could end up being a broken link in the future but 5.6 is a fixed reference to the published docs.