Why Does the FIO Performance Degrade in the Last 15s+(from 59w IOPS to 5w)When the FIO Is Executed for 1 Min?

[ssandl]

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fio commond:
fio -directory=/mnt/fio/ -direct=1 -iodepth 1 -thread -rw=randwrite -ioengine=psync -bs=8k -size=5G -numjobs=100 -runtime=60 -group_reporting -name=rand_100write_8k

At start:
IOPS: 61w
cpu%: 958
MEM:0.1%

last 15s：
IOPS：5w
CPU%：0
MEM：0.1%

I don't know why the fio CPU usage cannot increase in the last few seconds. In addition, the disk latency does not increase by 10 times, maybe increase by about 2 times.

[ssandl]

The disk is an NVMe disk and is not full.