plugins/slurm: Update README.md

## Installation

The plugin is written in C and requires the Slurm and the Scord C

The core of the plugin is written in C and requires the Slurm and the Scord C

libraries to be compiled. It also requires access to the Slurm

`spank.h` header. The plugin is compiled as a shared object that is

loaded by Slurm's plugin system.

command will compile the plugin:

```bash

  $ make slurm-plugin

  $ make slurm-plugin install

```

Once the shared object is compiled, it only needs to be copied

somewhere the Slurm plugin system can pick it up. For example, if

`libslurmadmcli.so` is installed in `/usr/local/lib/`, the following line

should be added to the Slurm plugin configuration file (usually

`/etc/slurm/plugstack.conf`)

Once the shared object is compiled, it only needs to be configured to be

used by Slurm. For example, if `libslurmadmcli.so` is installed in

`/usr/local/lib/`, the following line should be added to the Slurm plugin

configuration file (usually `/etc/slurm/plugstack.conf`)

```conf

  optional	/usr/local/lib/libslurmadmcli.so	scord_addr=ofi+tcp://127.0.0.1:52000 scord_proto=ofi+tcp://127.0.0.1 scordctl_bin=scord-ctl

The key-value pairs following the plugin are optional configuration

variables.

- `scord_addr`: The address to contact the Scord service in Mercury format

  (default to ofi+tcp://127.0.0.1:52000)

- `scord_addr`: The address to contact the Scord service in Mercury format.

  For instance, if the Scord service has been configured to listen on

  port `52000` on a machine with the IP address `192.168.1.128` and using

  `tcp` as the transport protocol, the address would be

  `ofi+tcp://192.168.1.128:52000`.

- `scord_proto`: The transport protocol to use in Mercury format (default

  to ofi+tcp).

  to `ofi+tcp`). Must be the same configured in Scord (see Scord's

  documentation for more details).

- `scordctl_bin`: The scord-ctl binary to run on every node of an

  allocation, can be the path to an executable (default to scord-ctl).

Besides the shared library, the plugin also installs

[prolog and epilog](https://slurm.schedmd.com/prolog_epilog.html)

scripts for job control under `$PREFIX/share/scord/slurm/`. In order to enable

them, the following lines should be added to Slurm's configuration file

(where `$PREFIX` should be replaced with the path where Scord is installed):

```conf

  Prolog=$PREFIX/share/scord/slurm/scord_prolog.sh

  Epilog=$PREFIX/share/scord/slurm/scord_epilog.sh

```

## Usage

The plugin extends Slurm's command line arguments to allow users to request

the deployment of adhoc storage services for their jobs. The following

arguments are available for `srun`/`sbatch`:

- `--adm-adhoc`: The job requires an adhoc storage service.

  The following types are supported:

    - `gekkofs`: The job requires the GekkoFS adhoc file system.

    - `expand`: The job requires the Expand adhoc file system.

    - `hercules`: The job requires the Hercules adhoc file system.

    - `dataclay`: The job requires the dataClay adhoc object store.

- `--adm-adhoc-local`: The adhoc storage service will be deployed on the

  same nodes as the compute nodes, but the adhoc nodes will not be shared

  with the application. This is the default mode.

- `--adm-adhoc-overlap`: The adhoc storage service will be deployed on the same

  nodes as the compute nodes and its nodes will be shared with the application.

- `--adm-adhoc-remote <adhoc_id>`: The job will use a remote and independent

  remote adhoc storage service that must have been previously deployed with the

  `--adm-adhoc-exclusive` option. An identifier for that adhoc storage service

  must be provided as an argument.

Users can request and control the automatic deployment of a remote adhoc

storage service using the following `srun`/`sbatch` arguments:

- `--adm-adhoc-exclusive`: The job allocation will be used exclusively for an

  adhoc storage service.

- `--adm-adhoc-nodes`: The number of nodes to use for the adhoc storage

  service. The nodes will be allocated from the same partition as the

  compute nodes. This option is only valid when used with

  `--adm-adhoc-local` or `--adm-adhoc-overlap`.

Examples:

```bash

# Request a local GekkoFS adhoc storage service

$ sbatch --adm-adhoc gekkofs [--adm-adhoc-local] script.sh

# Request a local (overlapping) GekkoFS adhoc storage service

$ sbatch --adm-adhoc gekkofs --adm-adhoc-overlap script.sh

# Request an independent GekkoFS adhoc storage service

$ sbatch --adm-adhoc gekkofs --adm-adhoc-exclusive --adm-adhoc-nodes 10 --adm-adhoc-walltime 00:10:00 noop.sh

Submitted batch job 42

Will deploy adhoc storage 123456

$ sbatch --adm-adhoc-remote 123456 --dependency=after:42 script.sh

```

## References

1. See manual page spank(7) and <slurm/spank.h>