reset search index in Exchange 2010 DAG
2- In the console tree, under Monitoring Tools, click Performance Monitor.
3- In the Performance Monitor pane, click Add (green plus sign).
4- In Add Counters, in the Select counters from computer list, select the server on which the mailbox database you want to monitor is located.
5- In the unlabeled box below the Select counters from computer list, select Full Crawl Mode Status in the MSExchange Search Indices performance object.
6- In the Instances of selected object box, select the instance for the user's mailbox database.
7- Click Add, and then click OK.
8- To make easier to monitor the Full Crawl Mode Status, right-click on graph screen then Properties.
9- Select Graph tab.
10- On View Combo box select Histogram bar and Maximum Vertical Scale insert 2, click OK.