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copied!<p>Quite large if your question is "what's the storage capacity of an mnesia database made up of a huge number of <code>disc_only_copies</code> tables" - you're largely limited by available disk space.</p> <p>An easier question to answer is what's the maximum capacity of a single mnesia table of different types. <code>ram_copies</code> tables are limited by available memory. <code>disc_copies</code> tables are limited by their <code>dets</code> backend (<a href="http://www.erlang.org/~hakan/mnesia_consumption.txt" rel="noreferrer">Hakan Mattsson on Mnesia</a>) - this limit is 4Gb of data at the moment.</p> <p>So the simple answer is that simple <code>disc_copies</code> table can store up to 4Gb of data before they run into problems. (<em>Mnesia doesn't actually crash if you exceed the on-disk size limit - the ram_copies portion of the table continues running, so you can repair this by deleting data or making other arrangements at runtime</em>)</p> <p>However if you consider other mnesia features, then the answer is more complicated.</p> <ul> <li><a href="http://erlang.org/doc/apps/mnesia/Mnesia_chap5.html#5.4" rel="noreferrer"><code>local_content</code></a> tables. If the table is a <code>local_content</code> table, then it can have different contents on each node in the mnesia cluster, so the capacity of the table is <code>4Gb * <number of nodes></code></li> <li><a href="http://erlang.org/doc/apps/mnesia/Mnesia_chap5.html#5.3" rel="noreferrer">fragmented tables</a>. Mnesia supports user configurable table partitioning or sharding using table fragments. In this case you can effectively distribute and redistribute the data in your table over a number of primitive tables. These primitive tables can each have their own configuration - say one ram_copies table and the rest disc_only_copies tables. These primitive tables have the same size limits as mentioned earlier and now the effective capacity of the fragmented table is <code>4Gb * <number of fragments></code>. (<em>Sadly if you fragment your table, you then have to modify your table access code to use <code>mnesia:activity/4</code> instead of <code>mnesia:write</code> and friends, but if you plan this in advance it's managable</em>)</li> <li><a href="http://code.google.com/p/mnesiaex/" rel="noreferrer">external copies</a> If you like living on the extreme bleeding edge, you could apply the <a href="http://code.google.com/p/mnesiaex/" rel="noreferrer">mnesiaex</a> patches to mnesia and store your table data in an external system such as <a href="http://code.google.com/p/erlaws/" rel="noreferrer">Amazon S3</a> or <a href="http://code.google.com/p/tcerl/" rel="noreferrer">Tokyo Cabinet</a>. In this case the capacity of the table is limited by the backend storage.</li> </ul>

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