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Uncle Block

An uncle block is a block that has been validated but has a lower level of difficulty than other blocks and can not be connected to the main chain. It is like a stale block or an orphan block in a bitcoin. Gavin Wood's Ethereum Yellow Paper says it's called Ommer.

If the number of uncle blocks increases, the following problems will occur.

  1. Delays transaction processing. If two or more miners are mined at the same time, two blocks are created, and the transactions in two blocks are different. At this time, the blocks attached to the main chain are processed immediately, but transactions in other blocks are not processed immediately.
  2. Resulting in wasted computing resources. When a block is created at the same time in Ethereum, the chain is split into two, and the longest chain is registered as the main chain. After all, the process of creating a short chain of uncle blocks is a waste of resources due to unnecessary computation.
  3. If the next block is generated after the uncle block is created, the average block creation time becomes longer, so the difficulty level after the block creation is reduced. As the difficulty level decreases, the block creation time is reduced and the impact of the miner with more computing power increases, so the data in the block can be changed maliciously by this miner. Since the total difficulty of the blockchain includes the difficulty of the uncle block, the sum of all difficulty levels also increases when an uncle block is added.

Reference - Core Ethereum Programming