"What does "expired data is filtered out and is not written back to the compacted StoreFile." mean?" Filtered data (by TTL) would be removed on compaction. That is what this statement means. As to your confusion from your test, remember that there is a difference between a "minor compaction" and a "major compaction". A "major compaction" is a re-writing of all files in a region, whereas a "minor compaction" is (possibly) a subset of the files in a Region. Because (I'm guessing) you actually mean that you've disabled schedule major compactions doesn't mean that compactions will never run in your system (this is actually a really idea if you've somehow done this, by the way). A minor compaction can remove data masked by a TTL -- this is the simple case. However, tombstones can *only* be removed when a major compaction runs. This is because the tombstone may be masking records that exist in a file which was not included in the compaction process. Long-story short: if you want to verify why your experiment works as it does, just grep out the region identifier for your table from the RegionServer log. You should see a message INFO (if not INFO, certainly at DEBUG) which informs you that a compaction occurred on that Region, the number of input files and size and the size of the output file. Before that compaction message, the file on disk would be the full size; after, it would be the reduced size. https://hbase.apache.org/book.html#compaction does a pretty good job explaining these nuances, but feel free to ask for more clarification.
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