I think we're deviating from the original thread quite a bit and I would never argue that in a production environment with plenty OSDs you should go for R=2 or K+1, so my example cluster which happens to be 2+1 is a bit unlucky. However I'm interested in the following On 11/16/20 11:31 AM, Janne Johansson wrote:
So while one could always say "one more drive is better than your amount", there are people losing data with repl=2 or K+1 because some more normal operation was in flight and _then_ a single surprise happens. So you can have a weird reboot, causing those PGs needing backfill later, and if one of the uptodate hosts have any single surprise during the recovery, the cluster will lack some of the current data even if two disks were never down at the same time.
I'm not sure I follow, from a logical perspective they *are* down at the same time right? In your scenario 1 up-to-date replica was left, but even that had a surprise. Okay well that's the risk you take with R=2, but it's not intrinsically different than R=3.