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Keeping the mutations coming from the streaming process as mutations like any other have a number of advantages - and that's why we do it. However, this makes it impossible for Seastar's I/O scheduler to differentiate between incoming requests from clients, and those who are arriving from peers in the streaming process. As a result, if the streaming mutations consume a significant fraction of the total mutations, and we happen to be using the disk at its limits, we are in no position to provide any guarantees - defeating the whole purpose of the scheduler. To implement that, we'll keep a separate set of memtables that will contain only streaming mutations. We don't have to do it this way, but doing so makes life a lot easier. In particular, to write an SSTable, our API requires (because the filter requires), that a good estimate on the number of partitions is informed in advance. The partitions also need to be sorted. We could write mutations directly to disk, but the above conditions couldn't be met without significant effort. In particular, because mutations can be arriving from multiple peer nodes, we can't really sort them without keeping a staging area anyway. Signed-off-by: Glauber Costa <glauber@scylladb.com>