Q:

Geowebcache disk quota is different from tile layers page

This is my disk quota page and following is my tile layers list page:

And I could not found my tiled images in server data directory. 

I think my tiles are not saving on disk. What is the problem that you think?

A:

I had a similar problem to this (Ubuntu 14.04, GeoServer 2.6.1). Geoserver/GeoWebCache was storing tiles in the temporary storage directory specified by the web app container (with the side-effect of the cache being emptied whenever the servlet was restarted).
I resolved this by:

Creating a new directory in the geoserver data directory (for me,
this was /var/lib/tomcat7/webapps/geoserver/data/gwc) - I ran into trouble setting it to locations outside of the geoserver directory.
Changing the permissions of the directory to be owned by tomcat (chown tomcat7:tomcat7 /var/lib/tomcat7/webapps/geoserver/data/gwc).
Adding the reference to the new directory to the geoserver web.xml (http://docs.geoserver.org/stable/en/user/geowebcache/config.html#changing-the-cache-directory)
Restarting tomcat (service tomcat7 restart)

Regarding the negative disk usage, I resolved this by removing the .db files from inside the diskquote_page_store directory (see https://jira.codehaus.org/browse/GEOS-5615)

