Why is the write successful AFTER the data is written to MemTable, isn’t the commit log sufficient (I imagine the write to MemTable can by asynchronous)?
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According to this blog
The main aim of the commit log is to track all data that has not been written to disk i.e. data in a memtable which has not been flushed to disk.
Commit-log is only considered as a backup of MemTable and only be used when restart a crashed server. Cassandra reads MemTable for unflushed data, not the commit-log. Therefore, if we ack after commit-log and before MemTable, the next read might not get the acked entry (not found).