Recent Orphaned Blocks Explanation 2 hours, 29 minutes
Orphaned blocks are natural part of Bitcoin network. Therefore, our recent orphan rate was probably - to a certain extent - a random noise. Since it has been higher than usual in the past few days, we have conducted a deep analysis of our internal infrastructure and made some adjustment.
Firstly, we have upgraded to Bitcoind 0.11.1 which definitely improved our mempool performance. However, the remaining problem was still in propagation of our blocks into the bitcoin network. Eventhough Slush Pool has been using the fast block relay network (
http://bitcoinrelaynetwork.org/) for over a year now, thorough analysis of block propagation times showed that our connections to the fast block relay network were not working properly in the last few weeks.
This connectivity issue, which affected all 9 bitcoin nodes that we currently operate, has been fixed with the help from Matt Corallo - the author of the fast block relay network. In addition, we have updated the relay network client to the latest version.
Since we have not experienced any orphaned block (invalid in Slush Pool terminology) in the last 2 days (or more than 30 consecutive blocks) we suppose the issue has been fixed. Moreover the block propagation times are now being carefully monitored and >we believe such increased attention will lead to stable validation in the future.
Finally we believe, the fast block relay network provides a very robust solution to this issue for all miners and mining pools.
Ending note:
Not only exchanges and wallets have been affected by persisting spam attack. Since pools depend on quick propagation times among Bitcoin network nodes, they were affected as well.
A very good analysis regarding this topic has been written and published on following website by Organ Ofcorti:
http://organofcorti.blogspot.cz/2015/10/october-18th-2015-mining-pool-statistics_21.htmAlthough he covered a few selected pools in his analysis there are other big players who were not analyzed and had higher rate of orphaned blocks in the weeks before as well. Therefore, it is safe to guess that the whole network was and is being affected to much higher extent.
As to why the current spam attack impacts orphan rates, we would like to point attention of our technically skilled miners to following statistics as well:
http://bitcoinrelaynetwork.org/stats.html 
