Details
Blocker Description
I recently ran into a couple data loss issues with small scans. Similar to HBASE-13262, these issues only appear when scans are configured in such a way that the max result size limit is reached before the caching limit is reached. As far as I can tell, this issue affects branches 0.98+
I should note that after investigation it looks like the root cause of these issues is not the same as HBASE-13262. Rather, these issue are caused by errors in the small scanner logic (I will explain in more depth below).
Furthermore, I do know that the solution from HBASE-13262 has not made its way into small scanners (it is being addressed in HBASE-13335). As a result I made sure to test these issues with the patch from HBASE-13335 applied and I saw that they were still present.
The following two issues have been observed (both lead to data loss):
1. When a small scan is configured with a caching value of Integer.MAX_VALUE, and a maxResultSize limit that is reached before the region is exhausted, integer overflow will occur. This eventually leads to a preemptive skip of the regions.
2. When a small scan is configured with a maxResultSize that is smaller than the size of a single row, the small scanner will jump between regions preemptively. This issue seems to be because small scanners assume that, unless a region is exhausted, at least 2 rows will be returned from the server. This assumption isn't clearly state in the small scanners but is implied through the use of skipRowOfFirstResult.
Again, I would like to stress that the root cause of these issues is NOT related to the cause of HBASE-13262. These issues occur because of inappropriate assumption made in the small scanner logic. The inappropriate assumptions are:
1. Integer overflow will not occur when incrementing caching
2. At least 2 rows will be returned from the server unless the region has been exhausted
I am attaching a patch that contains tests to display these issues. If these issues should be split into separate JIRAs please let me know.