Mnesia Table Fragmentation

How to start

Launch erl shell with mnesia:

erl -mnesia

Compile:

c(frags).

Create the tables and schema:

frags:create().

Start the observer, and inspect the "Table Viewer -> View/Mnesia Tables" menu:

observer:start().

You should see two frags. Try adding some sample data:

frags:populate(100).    

Now those tables shold have more data. Now add and remove some frags and examine the tables:

frags:add_frag().
frags:add_frag().
frags:del_frag().

Now delete all the frags by repeated issuing del_frag:

frags:del_frag().
...

Make sure only one table remains. Then disable the fragmentation for this table:

frags:deactivate().

At this point it becomes a regular table. We are no longer able to add fragments. Try this (what happends?):

frags:add_frag().

Take NOTE that adding data with populate will not always work now. This is because it still hashes (to frags) that don't exist. Use populate_no_frag to add data (which uses just vanilla transactions):

frags:populate_no_frag(100).

Now reactivate the fragmentation:

frags:activate().

And we can add frags again:

frags:add_frag().

References

Following these two resources:

• https://erlangcentral.org/wiki/index.php/Mnesia_Table_Fragmentation
• http://erlang.org/doc/apps/mnesia/Mnesia_chap5.html#id79999