I see a potential OOM, when a stream (e.g. repair) goes through the write path as it is with MVs.
StreamReceiveTask gets a bunch of SSTableReaders. These produce rowiterators and they again produce mutations. So every partition creates a single mutation, which in case of (very) big partitions can result in (very) big mutations. Those are created on heap and stay there until they finished processing.
I don't think it is necessary to create a single mutation for each partition. Why don't we implement a PartitionUpdateGeneratorIterator that takes a UnfilteredRowIterator and a max size and spits out PartitionUpdates to be used to create and apply mutations?
The max size should be something like min(reasonable_absolute_max_size, max_mutation_size, commitlog_segment_size / 2). reasonable_absolute_max_size could be like 16M or sth.
A mutation shouldn't be too large as it also affects MV partition locking. The longer a MV partition is locked during a stream, the higher chances are that WTE's occur during streams.
I could also imagine that a max number of updates per mutation regardless of size in bytes could make sense to avoid lock contention.
Love to get feedback and suggestions, incl. naming suggestions.