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nodetool tablestats: Partition keys number (estimate) in Scylla post migration from C* differs by 20% less up to 120% more than the original amount in C* #2545
I performed a migration of 3 KS, each with 1 table of ~10M partitions, in parallel, while utilizing 3 intermediate nodes, each has an NFS mount point to 1 of the C* nodes, to one of the KS.
After all sstables files loaded and compactions completed, the number of partitions it much bigger than the 9.8M we had in C*. So far in my tests the partition keys estimate post migration + compactions complete + nodetool flush, is wither 20% less up to 120% more than the original amount in C* 3.10.
@glommer wrote about the estimate:
Now let's look at what that metric really is in TableMetrics.java, it is basically:
And is also defined as an alias for EstimatedRowCount. The latter is what scylla-jmx responds to, and it translates to /column_family/metrics/estimated_row_count/
Looking at our implementation, we do not include memtables. Also, when
So my conclusion here is that Scylla is misreporting this. The fact
Another potential interesting difference is the calculation of the