I'm particularly interested in getting the tombstones to "promote" up the levels of LCS more quickly. Currently they get attached at the low level and don't propagate up to higher levels until enough activity at a lower level promotes the data. Meanwhile, LCS means compactions can occur in parallel at each level. So row tombstones in their own sstable could be up promoted the LCS levels preferentially before normal processes would move them up.
So if the delete-only sstables could move up more quickly, the compaction at the levels would happen more quickly. The threshold stuff is nice if I read 7019 correctly, but what is the % there? % of rows? % of columns? or % of the size of the sstable? Row tombstones are pretty compact being just the rowkey and the tombstone marker. So if 7019 is triggered at 10% of the sstable size, even a crapton of tombstones deleting practially the entire database would only be a small % size of the sstable. Since the row tombstones are so compact, that's why I think they are good candidates for special handling. On Tue, Feb 13, 2018 at 5:22 PM, J. D. Jordan <jeremiah.jor...@gmail.com> wrote: > Have you taken a look at the new stuff introduced by > https://issues.apache.org/jira/browse/CASSANDRA-7019 ? I think it may go > a ways to reducing the need for something complicated like this. > Though it is an interesting idea as special handling for bulk deletes. If > they were truly just sstables that only contained deletes the logic from > 7109 would probably go a long ways. Though if you are bulk inserting > deletes that is what you would end up with, so maybe it already works. > > -Jeremiah > > > On Feb 13, 2018, at 6:04 PM, Jeff Jirsa <jji...@gmail.com> wrote: > > > > On Tue, Feb 13, 2018 at 2:38 PM, Carl Mueller < > carl.muel...@smartthings.com> > > wrote: > > > >> In process of doing my second major data purge from a cassandra system. > >> > >> Almost all of my purging is done via row tombstones. While performing > this > >> the second time while trying to cajole compaction to occur (in 2.1.x, > >> LevelledCompaction) to goddamn actually compact the data, I've been > >> thinking as to why there isn't a separate set of sstable infrastructure > >> setup for row deletion tombstones. > >> > >> I'm imagining that row tombstones are written to separate sstables than > >> mainline data updates/appends and range/column tombstones. > >> > >> By writing them to separate sstables, the compaction systems can > >> preferentially merge / process them when compacting sstables. > >> > >> This would create an additional sstable for lookup in the bloom filters, > >> granted. I had visions of short circuiting the lookups to other > sstables if > >> a row tombstone was present in one of the special row tombstone > sstables. > >> > >> > > All of the above sounds really interesting to me, but I suspect it's a > LOT > > of work to make it happen correctly. > > > > You'd almost end up with 2 sets of logs for the LSM - a tombstone > > log/generation, and a data log/generation, and the tombstone logs would > be > > read-only inputs to data compactions. > > > > > >> But that would only be possible if there was the notion of a "super row > >> tombstone" that permanently deleted a rowkey and all future writes > would be > >> invalidated. Kind of like how a tombstone with a mistakenly huge > timestamp > >> becomes a sneaky permanent tombstone, but intended. There could be a > >> special operation / statement to undo this permanent tombstone, and > since > >> the row tombstones would be in their own dedicated sstables, they could > >> process and compact more quickly, with prioritization by the compactor. > >> > >> > > This part sounds way less interesting to me (other than the fact you can > > already do this with a timestamp in the future, but it'll gc away at > gcgs). > > > > > >> I'm thinking there must be something I am forgetting in the > >> read/write/compaction paths that invalidate this. > >> > > > > There are a lot of places where we do "smart" things to make sure we > don't > > accidentally resurrect data. Read path includes old sstables for > tombstones > > for example. Those all need to be concretely identified and handled (and > > tested),. >
