This article describes all forks that Sia went through, planned or unplanned.
On December 6th 2017 (Block 135,000) the Sia chain forked to apply an update to the consensus algorithm. Prior to this update the difficulty changes would be calculated using an overlapping adjustment period, this would cause the difficulty to start oscillating when a sudden change in hash rate happened. This update aims to prepare the Sia network for the future releases of ASICs.
The new algorithm would adjust the difficulty every block based on the timing of all blocks on the blockchain. This ensures that storage contracts will always start and end at the right time, regardless of hash rate changes.
But there was a bug in the new algorithm that caused to difficulty to drop sharply, eventually leading to dozens of blocks being mined per minute. The blocks could not propagate through the network quickly enough for all the pools to catch up, so siamining.com ended up mining all of them. Then the difficulty quickly rose up again landing on a stable 30 minutes per block rhythm.
Up to this point the blocks were still 30 minutes each. While this is inconvenient for transacting coins it was not a big problem for the network. The block rewards also dropped to 1/3 of what they should be so some miners left. On the 21st of January 2018 (Block 139,000) the network forked again to correct the algorithm back to 10 minute blocks. After the 21st the difficulty started rising rapidly because that same day Bitmain started sales of their Antminer A3 miners. Because of the huge inflation in hash rates the block times briefly dropped to 10 minutes before the algorithm could adjust enough to reach its desired timing.
Despite the 46 days of 30 minute blocks the network was still ahead of schedule compared to the global average of block times. So the difficulty briefly rose to 30 minutes again to compensate for the inaccuracies of the previous difficulty algorithm.
Early June 2018 community member Mark (known as 'toaster' at that time) announced that he would fork the Sia codebase and start his own project based on Sia called Hyperspace with a new currency called Space Cash (XSC). On the first of July all addresses on the Sia blockchain would be credited one tenth of the amount of siacoins they held in Space Cash. The total size of the airdrop was 3.5 billion Space Cash. A snapshot of the Sia blockchain was done on block 161,358 for a 10:1 (SC:XSC) ratio airdrop.
At the launch of the network an additional 600 million Space Cash was generated and distributed among developers and contributors to the Sia project. Developers NickH (Luxor mining), darval (Open source mining pool), RBZL (siasetup.info), Fornax (pixeldrain.com), drexel (siadrive), tbenz9 (Sia developer / documenter), hakkane (siastats.info), sebdude (siaberry), bryan (easysia.com) and atreides (Hyperspace contributor) received 54 million time locked Space Cash. Contributors CryptoHaag, moonshot, Wibs, Aura89, MasterHW and Spacing Radar101 received 10 million time locked Space Cash. These users are now all moderators in the Hyperspace discord server. The distributed coins would be locked until Hyperspace block 157,680 which should be mined roughly three years after the genesis of the blockchain.
After a long period of debate it was concluded that the best solution to the Obelisk debate was to slightly modify the nonce of the mining algorithm in order to fork all competing ASICs off the network. Starting from Sia version 1.3.6 the Obelisk SC1 will be the only miner able to mine Sia at profitable hash rates until other manufacturer release new competing ASICs. Version 1.3.5 was released for the people who would prefer to stay on the other side of the fork compatible with all ASIC miners. However, the developer overlooked the impact of the reduced hashrate and does not include a difficulty adjustment on version 1.3.6, potentially causing a very long blocktime after fork so version 1.3.7 was released 9 days before the expected fork date. The fork had been activated on block 179,000, which took about 14.6 hours to be mined because of a bug in the difficulty adjustment algorithm, causing it to need a block to be mined before the difficulty adjustment kicks in to significantly drop the difficulty. The main chain continues with Sia version 1.3.7 and above.