huaxiang sun commented on HBASE-17172:

One more question for you, Jingcheng, :). When threshold is so big that size 
for all mob files is less than this threshold, in this case, if there are _del 
files, the minor mob compaction actually turns into a major mob compaction. 
What is the reason behind the design? Since threshold is a user configurable 
variable, user may choose to configure a large value and turns the mob 
compaction into a major one, if there are _del files, compaction will take 
longer than expected. Thinking about compacting 1 mob file with _del files only 
for major_mob_compact case so user is aware of what is going to happen. 
comments? Thanks..

> Optimize major mob compaction with _del files
> ---------------------------------------------
>                 Key: HBASE-17172
>                 URL: https://issues.apache.org/jira/browse/HBASE-17172
>             Project: HBase
>          Issue Type: Improvement
>          Components: mob
>    Affects Versions: 2.0.0
>            Reporter: huaxiang sun
>            Assignee: huaxiang sun
> 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.

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