Time series data model and tombstones

[John Sanda]

I have a time series data model that is basically:

CREATE TABLE metrics (
    id text,
    time timeuuid,
    value double,
    PRIMARY KEY (id, time)
) WITH CLUSTERING ORDER BY (time DESC);

I do append-only writes, no deletes, and use a TTL of seven days. Data
points are written every seconds. The UI queries data for the past hour,
two hours, day, or week. The UI refreshes and executes queries every 30
seconds. In one test environment I am seeing lots of tombstone threshold
warnings and Cassandra has even OOME'd. Since I am storing data in
descending order and always query for recent data, I do not understand why
I am running into this problem.

I know that it is recommended to do some date partitioning in part to
ensure partitions do not grow too large. I already have some changes in
place to partition by day.. Before I make those changes I want to
understand why I am scanning so many tombstones so that I can be more
confident that the date partitioning changes will help.

Thanks

- John