Lmod on Shared Home File Systems

Many sites have a single Operating System and one set of modules across their cluster. If a site has more than one cluster, they may chose to have a separate home directory for each cluster. Some sites may wish to have multiple clusters share a single home directory. While this strategy has some advantages, it complicates things for your users and adminstrators. If your site has a single home directory sharing between two or more clusters, you have a shared home file system.

As a further complication, your site may or may not have a shared home file system even if you have two or more clusters. If you have separate login nodes for each cluster then you do have a shared home file system. If you have a single login which can submit jobs to different clusters then you do not have a shared home file system.

The way to think about this is each cluster is going to have at least some modules which are different. Module collections need to be unique to each cluster. The trick described below will make them unique for each cluster.

Sites that use a shared home file system across multiple clusters should take some extra steps to ensure the smooth running of Lmod. Typically each cluster will use different modules.

There are three steps that will make Lmod run smoothly on a shared home filesystem:

  1. It is best to have a separate installation of Lmod on each cluster.

  2. Define the environment variable “LMOD_SYSTEM_NAME” uniquely for each cluster.

  3. If you build a system spider cache, then build a separate cache for each cluster.

A separate installation on each cluster is the safest way to install Lmod. It is possible to have a single installation but since there is some C code build with Lmod, this has to work on all clusters. Also the location of the Lua interpreter must be exactly the same on each cluster.

It is also recommended that you set “LMOD_SYSTEM_NAME” outside of a modulefile. It would be bad if a module purge would clear that value. When you set this variable, it makes the module collections and user spider caches unique for a given cluster.

A separate system spider cache is really the only way to go. Otherwise a “module spider” will report modules that don’t exist on the current cluster. If you have a separate install of Lmod on each cluster then you can specify the location of system cache at configure time. If you don’t, you can use the “LMOD_RC” environment to specify the location of the lmodrc.lua file uniquely on each cluster.

Lmod knows about the system spider cache from the lmodrc.lua file. If you install separate instances of Lmod on each cluster, Lmod builds the scDescriptT table for you. Otherwise you can modify lmodrc.lua to point to the system cache by adding scDescriptT to the end of the file:

scDescriptT = {
    ["dir"] = "<location of your system cache directory>,
    ["timestamp"] = "<location of your timestamp file",

where you have filled in the location of both the system cache directory and timestamp file.