The IRIS Installation Guide for Linux, Installation Directory section, says "Do not choose the /home directory, any of its subdirectories, or the /usr/local/etc/irissys directory." but there are no suggestions or any default.

What are your opinions on this? For example, I see that IRIS in a Docker container is installed in /usr/irissys. I'm wondering why that directory was chosen.

The official Linux filesystem docs say:

• /usr is the second major section of the filesystem. /usr is shareable, read-only data. That means that /usr should be shareable between various FHS-compliant hosts and must not be written to. Any information that is host-specific or varies with time is stored elsewhere. Large software packages must not use a direct subdirectory under the /usr hierarchy.
• The /usr/local hierarchy is for use by the system administrator when installing software locally. It needs to be safe from being overwritten when the system software is updated. It may be used for programs and data that are shareable amongst a group of hosts, but not found in /usr.
• /opt is reserved for the installation of add-on application software packages. A package to be installed in /opt must locate its static files in a separate /opt/<package> or /opt/<provider> directory tree, where <package> is a name that describes the software package and <provider> is the provider's LANANA registered name.

From that, it seems like /usr/local/InterSystems/<instance> or /opt/InterSystems/<instance> would be "right." To me that seems better than /usr/irissys, /usr/irissys1, /usr/irissys2, etc.