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Sylvain Lebresne commented on CASSANDRA-9420:
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bq. The problem stems from the fact that Cassandra plays safe when a TTL has
expired, and turns it into a tombstone, rather than getting rid of it on the
spot.
If {{gcGrace == 0}}, which you seem fine with, then we'll get rid of expired
column "on the spot". The fact that expired column are turned into tombstones
is a technicality that only exists so we keep expired columns for gcGrace
seconds after expiration, but if the later is 0, it's removed right away. If
you've observed differently in practice, then that would sound like a bug and
if you provided reproduction steps we certainly can look into it. But, and
unless I've misunderstood what you're saying, we don't need yet another option
for this.
> Table option for promising that you will never touch a column twice
> -------------------------------------------------------------------
>
> Key: CASSANDRA-9420
> URL: https://issues.apache.org/jira/browse/CASSANDRA-9420
> Project: Cassandra
> Issue Type: New Feature
> Reporter: Björn Hegerfors
>
> There are time series use cases where you write all values with various TTLs,
> have GC grace = 0 and never ever update or delete a column after insertion.
> In the case where all TTLs are the same, DTCS with recent patches works
> great. But when there is lots of variations in TTLs, you are forced to choose
> between splitting your table into multiple TTL tiers or having your SSTables
> filled to the majority with tombstones. Or running frequent major compactions.
> The problem stems from the fact that Cassandra plays safe when a TTL has
> expired, and turns it into a tombstone, rather than getting rid of it on the
> spot. The reason is that this TTL _may_ have been in a column which has had
> an earlier write without (or with a higher) TTL. And then that one should now
> be deleted too.
> I propose that there should be table level setting to say "I guarantee that
> there will never be any updates to any columns". The effect of enabling that
> option is that all tombstones and expired TTLs should always be immediately
> removed during compaction. And the check for dropping entirely expired
> SSTables can be very loosened for these tables.
> This option should probably require gc_grace_seconds to be set to zero. It's
> also questionable if writes without TTL should be allowed to such a table,
> since those would become constants.
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