Similar question has previously come over
HDFS-3091 and friends, but the essential problem is: "Why can't I write to my cluster of 3 nodes, when I just have 1 node available at a point in time.".
The policies cover the 4 options, with Default being default:
Disable -> Disables the whole replacement concept by throwing out an error (at the server) or acts as Never at the client.
Never -> Never replaces a DN upon pipeline failures (not too desirable in many cases).
Default -> Replace based on a few conditions, but whose minimum never touches 1. We always fail if only one DN remains and none others can be added.
Always -> Replace no matter what. Fail if can't replace.
Would it not make sense to have an option similar to Always/Default, where despite trying, if it isn't possible to have > 1 DN in the pipeline, do not fail. I think that is what the former write behavior was, and what fit with the minimum replication factor allowed value.
Why is it grossly wrong to pass a write from a client for a block with just 1 remaining replica in the pipeline (the minimum of 1 grows with the replication factor demanded from the write), when replication is taken care of immediately afterwards? How often have we seen missing blocks arise out of allowing this + facing a big rack(s) failure or so?