Hello Deepak, The dev@cassandra list is exclusive for development announcements and discussions, so I will reply to users@cassandra as someone else might have a similar question.
Basically, there is pre-check, that defines which sstables are eligible for single-sstable tombstone compaction, and an actual check that determines if a key is present in a single sstable before performing the actual tombstone removal (otherwise it doesn't do anything). The pre-check is cheap, and only consider an sstable eligible for tombstone compaction if the sstable key range does not overlap with any other sstable key range. The actual check might need to read from disk (=more seeks on spindles) to confirm if the sstables with range overlap actually contain the tombstoned key, in order to define if it's safe or not to drop the tombstone. In the case of size tiered compaction, it's common for many sstables to have overlapping ranges, so the tombstone compaction is almost never triggered, so you will need to wait until compactions organically remove tomsbtones. The unchecked_tombstone_compaction removes the pre-check for overlapping ranges, but still performs the check to determine if it's safe to drop a tombstone. So, it's possible that your I/O will increase if you enable this property, since more data will need to be read to perform the actual checks, but otherwise it's safe to use this setting. A good way to check if the setting is useful is to watch your droppable tombstone ratio metrics after enabling it. Cheers, Paulo 2015-10-14 23:27 GMT-07:00 Deepak Nagaraj <n.dee...@gmail.com>: > Hi Paulo, C* devs, > > I have a question on "unchecked_tombstone_compaction" option. I > understand that setting this to true prevents a heuristic check on keys > that span multiple sstables. > > But I also read that the heuristic was introduced because not having it > can cause resurrections (i.e. sstable1 may have data, sstable2 may have > tombstone, and when we delete the tombstone, deleted data suddenly shows > up). > > So - isn't setting unchecked_tombstone_compaction to "true" a dangerous > setting? Won't it cause resurrections? What is the use case for this > knob, and when do I know I can set it to true safely? > > I've read the source code, Jira 6563, and relevant e-mail threads many > times but I still don't have a clear understanding. > > Thanks in advance, > -deepak > >
