With the files getting large its to do with the product defaults, so up to 10.15 a single file was appended to until the spindle size is reached then a new spindle created, so to reach large files sizes is just a factor of message content size and spindle size, that is why I suggested earlier to lower it to something like 1000 given the exceptionally large file size being reached. In 10.15+ there is an extra measure which will use file size or spindle size to decide to complete appending to a spindle and create a new one.
Queues both Mixed and Persistent will write to the disk always in all releases as we need to have durability whilst transacting the event and do not depend on consumers to hold events, channels(Topics) if they have no consumers and have the JMS engine will drop events and not write to disk as they do not have such semantics.
Yes, as I stated first response, lowering the spindle size will resolve this issue given all the information we have been provided and this kind of problems is why we added a feature to 10.15 to stop this occuring
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