Dear Toei krub,
The DNS logging feature will send the logs externally to a syslog server and it is being sent by the PE thus wont have too much impact on mgmt. CPU.
This feature be GAed with upcoming Ion release (NetScaler 11.0), tentatively in end of June (not get confirm exact date yet).
Please feel free to let me know any enquiry/support from you krub.
Thank you.