Details
-
Bug
-
Status: Closed
-
Major
-
Resolution: Duplicate
-
None
-
None
-
None
Description
Given the potentially low and misleading value of the metric, and how much effort must be expended to collect them, I would argue at least we should allow users to disable the feature completely.
The first problem is the data the metric delivers is not very useful. On any given busy system, this value is often 100%. On a sample system here, 12% of the tables were at either 0 or 100%. Furthermore the 100% metric is not particularly informative. If a table has 100% 'fragmentation' it does not necessarily imply that this table is in dire need of compaction. The HBase compaction code will generally keep at least 2 store files around - it refuses to minor compact older and larger files, preferring to merge small files. Thus on a table taking writes on all regions, the expected value of fragmentation is in fact 100%. And this is not a bad thing either. Considering that compacting a 500GB table will take an hour and hammer a cluster, misleading users into striving to get to 0% is non ideal.
The other major problem of this feature is collecting the data is non-trivial on larger clusters. I did a test where I did a lsr on a hadoop cluster, and to generate 15k lines of output, it pegged the namenode at over 100% cpu for a few seconds. On a cluster with 7000 regions, we can clearly easily have 14,000 (2 store files per region is typical) files thus causing spikes against the namenode to generate this statistic.
I would propose 3 courses of actions:
- allow complete disablement of the feature, including the background thread and the UI display
- change the metric to mean '# of regions with > 5 store files'
- replacing the metric with a completely different one that attempts to capture the spirit of the intent but with less load.