Check the V2R8 documentation, it has clearly presented that the S5720HI does not support VLAN mirroring.

Root Cause

Solution
Use ACL to do the mirroring instead of VLAN mirroring
[Switch] acl number 3000
[Switch-acl-adv-3000] rule permit tcp source 10.1.1.0 0.0.0.255
[Switch]traffic-mirror vlan 30 inbound acl 3000 to observe-port 1