This morning in the train (of all places) I realized a fundamental issue in how KEEP_DELETED_CELLS is implemented.
The problem is around knowing when it is safe to remove a delete marker (we cannot remove it unless all cells affected by it are remove otherwise).
This was particularly hard for family marker, since they sort before all cells of a row, and hence scanning forward through an HFile you cannot know whether the family markers are still needed until at least the entire row is scanned.
My solution was to keep the TS of the oldest put in any given HFile, and only remove delete markers older than that TS.
That sounds good on the face of it... But now imagine you wrote a version of ROW 1 and then never update it again. Then later you write a billion other rows and delete them all. Since the TS of the cells in ROW 1 is older than all the delete markers for the other billion rows, these will never be collected... At least for the region that hosts ROW 1 after a major compaction.
Note, in a sense that is what HBase is supposed to do when keeping deleted cells: Keep them until they would be removed by some other means (for example TTL, or MAX_VERSION when new versions are inserted).
The specific problem here is that even as all KVs affected by a delete marker are expired this way the marker would not be removed if there just one older KV in the HStore.
I don't see a good way out of this. In parent I outlined these four solutions:
So there are three options I think:
- Only allow the new flag set on CFs with TTL set. MIN_VERSIONS would not apply to deleted rows or delete marker rows (wouldn't know how long to keep family deletes in that case). (MAX)VERSIONS would still be enforced on all rows types except for family delete markers.
- Translate family delete markers to column delete marker at (major) compaction time.
- Change HFileWriterV* to keep track of the earliest put TS in a store and write it to the file metadata. Use that use expire delete marker that are older and hence can't affect any puts in the file.
- Have Store.java keep track of the earliest put in internalFlushCache and compactStore and then append it to the file metadata. That way HFileWriterV* would not need to know about KVs.
And I implemented #4.
I'd love to get input on ideas.