2017-Aug-14 17:08:19.522518 INFO ----- BLOCK ADDED AS ALTERNATIVE ON HEIGHT 8168
2017-Aug-14 17:08:19.522518 INFO id: 2dcd21ebab0d61aea568baa703ba97422e497194b7e56b51f57ec5e8f1cfc7f8
2017-Aug-14 17:08:19.522518 INFO PoW: 01aeb5a1d8a121e12baa6e35c0dd9d2ec05c07b5e87fee428256727124000000
2017-Aug-14 17:08:19.522518 INFO difficulty: 1729782
2017-Aug-14 17:15:39.060385 INFO ###### REORGANIZE on height: 8168 of 8168 with cum_difficulty 12157597779
2017-Aug-14 17:15:39.060385 INFO alternative blockchain size: 2 with cum_difficulty 12159324447
2017-Aug-14 17:15:39.257911 INFO ----- BLOCK ADDED AS ALTERNATIVE ON HEIGHT 8168
2017-Aug-14 17:15:39.257911 INFO id: 68a93dd3a8c62d81d9174fa12ab2956d3d67d526e5354d980b6072c10e4dc157
2017-Aug-14 17:15:39.257911 INFO PoW: 73c7bb78024334fb241641780be15537a87ca3d31c1a215e784a14b2b7050000
2017-Aug-14 17:15:39.257911 INFO difficulty: 1729782
2017-Aug-14 17:15:39.273953 INFO REORGANIZE SUCCESS! on height: 8168, new blockchain size: 8170
What is this?
Quoting from the net
https://monero.stackexchange.com/questions/2018/block-added-as-alternative-on-height-xxxxxReorgs happen when there is temporarily a disagreement about what the longest chain is. It happens in every Cryptocurrency, and the shorter the block time, the more frequent it happens. Basically, picture a guy in the Philippines and a guy in Spain each mining what they say is block 780000 at almost exactly the same time. Half the network sees the one guys block first and accepts it, while rejecting the other guys block. The other half the network does the opposite. It's actually not that uncommon for this to happen, but usually someone mining on one of those two chains finds another block and adds it on, and then everyone goes "oh look, that chain is longer" and ditches the other one. BUT, sometimes, the near tie in finding blocks can happen two or three or four times in a row. Eventually, one side wins out, and anyone on the wrong chain has to reorganize. This is why merchants are recommended to wait for a few confirmations before fully crediting a payment for big ticket items. The more confirmations (or the more blocks that get added on top of the one your transaction is in), the harder it is to happen on a reorg. The whole problem is caused by a slight delay in sending signals to all corners of the earth that a miner found a block. The total lag is pretty much ~15 seconds or so. The shorter the block time, the larger an effect that lag has. i.e. for a 1 minute block, that lag is 1/4th the block time, but for a 2 minute block, it is 1/8th the block time. In bitcoin's case, it is 1/40th the block time. The smaller that fraction gets, the less likely a reorg is to occur.