I see sharding and swarms are in the IOTA roadmap. The question is why is sharding necessary? Can it be compared to the way Ethereum plans to shard?
The only info I managed to find was a paragraph on the IOTA blog which is vague.
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Sharding is the process of dividing up data. For example, if you had a database containing information about all of the people alive today it could be quite large (depending on what information you store about each person).
Sharding lets you divide the dataset up across machines based on some criteria. For example, you could set up X shards and choose which shard a record goes to based on the year the person was born.
When you want to list everyone named "Smith" then each of the shards is contacted and told to return a list of IDs of people named "Smith" - the client (your computer) then puts the results from all shards together to give you a complete picture (all Smiths from all shards) - but showing only the data you asked for (say ID, GivenName, FamilyName, Title) so that it fits into memory.