GEOM framework:

Geom is a framework that makes it possible to stack different functionalities for storage devices.
The diagram below shows how classes can be stacked as provider/consumer to add functionalities to storage devices:

Geom modules(classes)

Different classes are loaded into the kernel to add new functionalities to storage devices.
GEOM allows the layering of these components(classes). GEOM modules or classes add features to storage devices in a ”stackable” way.
The output of each module can be used as the input of the next module in a producer/consumer relationship.

Loading the GEOM modules:

You can load these modules into the kernel using the below:

We can also load them at boot time by mentioning it in /boot/loader.conf.

GEOM commands:

These commands such as geomglabelgmirror and graid3 help us access the functionalities of different modules(classes) in the GEOM framework , 
Each  module(layer) has specific commands related to it.

Example of GEOM commands(RAID3):

We can access the functionalities of these layers either through the generic geom command such as:

Or directly through the command specific to the layer, in this case, the RAID-3 module(class) :

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