Reference: Paging and Unacknowledged Messages
24.9. Paging and unacknowledged messages Note Please note that until messages are acknowledged they are still in memory on the server, so they contribute to the size of messages on a particular address. If messages are paged to disk for an address, and are being consumed, they will be depaged from disk when enough memory has been freed up in that address after messages have been consumed and acknowledged. However if messages are not acknowledged then more messages will not be depaged since there is no free space in memory. In this case message consumption can appear to hang. If not acknowledging explictly messages are acknowledged according to the ack-batch-size setting. Be careful not to set your paging max size to a figure lower than ack-batch-size or your system may appear to hang!