Setting defrag-lwm-pct to 75% will mean that aerospike will defrag blocks that are only 25% full. Typically you always want the LWM to be the same as the HWM. FAQ: Defragmentation
Basically, aerospike will only write to ‘free-blocks’ and blocks can only be free once they have been defragmented and made available. This parameter is usually set to 50% along with the high-water-mark-disk-pct. Higher values generate more write amplification by defragging more aggressively - lower values will save on IO but if you have high amounts of disk used or high write thoughput then the avail will drop.
Why did you set defrag-lwm to 75%? What’s most likely happenning is that its queuing up more defrag than it can process. You can find out by looking from ‘defrag-q’ in the aerospike log/journal.
If defrag is not keeping up, lowering defrag sleep should be first. Increasing defrag-lwm has write amplification effects so its best not to raise that unless there is no other option - assuming you have the IO for it.