In the documentation for expiring data for Cassandra (here) it is mentioned that
Expiring data has an additional overhead of 8 bytes in memory and on disk (to record the TTL and expiration time) compared to standard data.
If one sets a TTL (time-to-live) on a table level, does that mean that for each data entry there is an overhead of 8 bytes more in memory and on disk multiplied by the number of columns, or it's independent of the number of columns?
For example, in the documentation one also finds the example here to determine the TTL for a column, even though data is inserted on more than 1 column and TTL is defined for the actual data entry being inserted, not on a per-column basis.
No, not anymore at least. That documentation is outdated and only relevant pre 3.0.
Currently if all the columns in a partition or a row in a partition have same TTL set at insertion it is just set the once for it. If they are stored they are written delta encoded from sstables minTimestamp as an unsigned variable int, not 8 bytes.