Today, when there is a _del file in mobdir, with major mob compaction, every mob file will be recompacted, this causes lots of IO and slow down major mob compaction (may take months to finish). This needs to be improved. A few ideas are:
1) Do not compact all _del files into one, instead, compact them based on groups with startKey as the key. Then use firstKey/startKey to make each mob file to see if the _del file needs to be included for this partition.
2). Based on the timerange of the _del file, compaction for files after that timerange does not need to include the _del file as these are newer files.