We occasionally see that the under-replicated block count is not going down quickly enough. We've made at least one fix to speed up block replications (
HDFS-9205) but we need better insight into the current state and activity of the block re-replication logic. For example, we need to understand whether is it because re-replication is not making forward progress at all, or is it because new under-replicated blocks are being added faster.
We should include additional metrics:
- Cumulative number of blocks that were successfully replicated.
- Cumulative number of re-replications that timed out.
- Cumulative number of blocks that were dequeued for re-replication but not scheduled e.g. because they were invalid, or under-construction or replication was postponed.
The growth rate of of the above metrics will make it clear whether block replication is making forward progress and if not then provide potential clues about why it is stalled.
Thanks Arpit Agarwal for the offline discussions.