The write path of an update
Inserting a duplicate primary key is treated as an upsert. Eventually, the updates are streamed to disk using sequential I/O and stored in a new SSTable.
Columns are overwritten only if the timestamp in the new version of the column is more recent than the existing column, so precise timestamps are necessary if updates (overwrites) are frequent. The timestamp is provided by the client, so the clocks of all client machines should be synchronized using NTP (network time protocol), for example.