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Bitcoin ABC (BCHA) Deep Reorganisation incident 29 March 2021
On 29 March 2021, Bitcoin ABC experienced a deep reorg. The following is a reconstruction of events, based on blockchain data.
Due to its low price compared to other bitcoin forks, the BCHA network has a rather low hashrate compared to the overall double-SHA256 mining market. A large majority of hashrate is controlled by one single pool: ZULUPooL. ZULUPooL is a merge-mining pool, which mines BCHA simultaneously with Hathor.
Like many blocks, block 679554 was merged-mined by ZULUPooL. However, from block 679555 onward, it appears that ZULUPooL had trouble with their merged-mining setup, and switched to an emergency non-merged-mining mode where it would just mine BCHA without mining Hathor. In this emergency mode, no transactions were included in the BCHA blocks, other than the coinbase transaction.
After a string of 13 empty blocks, all mined by ZULUPooL, ZULUPooL mined a second block at height 679555. Apparently their normal mining setup worked again: this block was merged-mined with Hathor and contains 15 BCHA transactions. The new block 679555 then got three confirmations from Mining-Dutch and one from ViaBTC. ZULUPooL then shifted more hashrate from the old chain to the new chain, even though it also kept mining the old chain in the supposed emergency mode.
Mining on the old chain completely stopped after 457 post-split blocks, so eventually the new chain is going to overtake the old chain in both block height and proof-of-work.
There was some speculation that the split was caused by ABC's reorg protection mechanism escalating a block race into an accidental chain split, but this is clearly not what happened. Instead, miners collectively decided to abandon a dozen blocks (which later became hundreds), for reasons we can only speculate. A possible reason is a community dislike of long strings of empty blocks, which led to a similar decision to perform a deep reorg in November 2020.
In conclusion, the reorg protection features worked as planned: they rejected the new chain replacing the old chain. However, reorg protection is obviously not going to be helpful if in fact BCHA's community repeatedly desires and approves deep reorgs.