The expired_objects metric is the number of objects expired from this namespace on this node since the server started. I.E. expired_objects are the number of objects that have been removed via the Namespace SUPervisor (NSUP) thread’s periodic scan.
The defrag-lwm-pct config triggers a write-block be enqueued for defragmentation when the writeblock is depleted below the value of defrag-lwm-pct. This means that, in the worst case, the device could be defrag-lwm-pct full and have zero device_available_pct. Use caution when increasing the defrag-lwm-pct as it has a non linearly increasing write-amplification effect. I recommend reading more about the defragmentation process and about difference between availible_percent and free_precent.
Based on the description, I believe you are likely not sized for the worst-case disk utilization for your configuration. You may want to allocate more of your SSD to this namespace. You could also lower the high-water-disk-pct config to be closer to the configured defrag-lwm-pct to ‘early-expire’ records which will encourage device write-blocks to become eligible for defragmentation. To be certain, could you share the oputput of:
asadm -e "show stat namespace like 'file|device'"