# "Configless" Slurm

"Configless" Slurm is a feature that allows the compute nodes — specifically the slurmd process — and user commands running on login nodes to pull configuration information directly from the slurmctld instead of from a pre-distributed local file. Your cluster does require a central set of configuration files on the Slurm controllers — "configless" in Slurm's parlance means that the compute nodes, login nodes, and other cluster hosts do not need to be deployed with local copies of these files.

The slurmd on startup will reach out to a slurmctld that you specify and the config files will be pulled to the node. This slurmctld can be identified by either an explicit option, or — preferably — through DNS SRV records defined within the cluster itself.

If you have a login node you will be running client commands from, those client commands will have to use the DNS record to get the configuration information from the controller when they run. If you expect to have a lot of traffic from a login node, this can generate a lot of requests for the configuration files. In cases like this, you may want to consider running slurmd on the machine so it can manage the configuration files, but not allowing it to run jobs.

## Installation

There are no extra steps required to install this feature. It is built in by default starting with Slurm 20.02.

## Setup

The slurmctld must first be configured to run in the configless mode. This is handled by setting SlurmctldParameters=enable_configless in slurm.conf and restarting slurmctld.

Once enabled, you must configure the slurmd to get its configs from the slurmctld. This can be accomplished either by launching slurmd with the --conf-server option, or by setting a DNS SRV record and ensuring there is no local configuration file on the compute node.

The --conf-server options takes precedence over the DNS record.

The command line option takes "$host[:$port]", so an example would look like:

    slurmd --conf-server slurmctl-primary:6817

Specifying the port is optional and will default to 6817 if it is not present.

The same information can be provided in a DNS SRV record. For example:

    _slurmctld._tcp 3600 IN SRV 10 0 6817 slurmctl-backup
    _slurmctld._tcp 3600 IN SRV 0 0 6817 slurmctl-primary

Will provide the required information to the slurmd on startup. As shown above, multiple SRV records can be specified if you have deployed Slurm in an HA setup. The DNS SRV entry with the lowest priority should be your primary slurmctld, with higher priority values for backup slurmctlds.

## Initial Testing

With the slurmctld configured and slurmd started, you can check in a couple places to make sure the configs are present on the node. Config files will be in SlurmdSpoolDir under the /conf-cache/, and a symlink to this location will be created automatically in /run/slurm/conf. You can confirm that reloading is working by adding a comment to your slurm.conf on the slurmctld node and running scontrol reconfig and checking that the config was updated.

## Limitations

Using "%n" in "SlurmdSpoolDir" or "SlurmdPidFile" will not be properly substituted for the NodeName unless slurmd is also launched with the "-N" option.

If you are using systemd to launch slurmd, you must ensure that "ConditionPathExists=*" is not present in the unit file or the slurmd will not start. (The example slurmd.service file shipped in Slurm 20.02 and above does not include this entry.)

If any of the supported config files "Include" additional config files, the Included configs will NOT be shipped to the slurmds. Any additional config files will need to be shared a different way or added to the parent config.

## Notes

The order of precedence for determining what configuration source to use is as follows:

1. The slurmd --conf-server $host[:$port] option
2. The -f $config_file option
3. The SLURM_CONF environment variable (if set)
4. The default slurm config file (likely /etc/slurm.conf)
5. Any DNS SRV records (from lowest priority value to highest)

Supported configuration files are:

- slurm.conf
- acct_gather.conf
- cgroup.conf
- cgroup_allowed_devices_file.conf
- ext_sensors.conf
- gres.conf
- job_container.conf
- knl_cray.conf
- knl_generic.conf
- plugstack.conf
- topology.conf

Last modified 11 March 2021