Thank you for your reply! Indeed! After updated to 6.9 RC2, everything works perfectly fine. I spent a lot of time debugging this.
Don't know whether it's a problem of older version of unRaid or Docker. I was trying out unRaid, but disappointed to see no response from unRaid in your thread. Also I found there's some other io problem with unRaid Docker running qBittorrent. I already switched to other NAS OS. But thank you very much for your reply!