SharePoint 2013 - Search Index partition error - a yellow triangle

If something goes wrong with SP 2013 or your Search you will see Search Index partition error. This error some times comes when we restore Search Databases too. This means there is something wrong with the index partition on that particular search server.

I have seen many post and resolution on resolving the SharePoint Search Index partition error - a yellow triangle. I would recommend following solution before you can try to reset Index or even try to re-create the Search Service application from scratch. Definitely when it comes to Production both the solution stated above will be time consuming and lengthy downtime if you have millions of items to be crawled.

Try following:

1. Log on to the Search Server where partition has error.
2. Clear the SharePoint Server Cache. 
1. Stop SharePoint Timer Service.
2. Browse to %SystemDrive%\ProgramData\Microsoft\SharePoint\Config
   Note: ProgramData folder is a hidden folder.
3. Select the Guid folder which was recently modified.
4. Select and delete all the XML files except the Cache.ini file.
5. Open and Edit the Cache.ini file and set it to 1.
6. Start SharePoint Timer Service.
7. Verify the XML files start populating again. wait for few minutes
   Note: You can compare the number of files from another healthy server in your farm else wait for few minutes till all files are recreated.
3. Restart the "SharePoint Server Search 15" service in Services.msc
4. Wait for few more minutes and you wont see a yellow triangle - a partition error.
   Note : You should see the Search Index folder growing in its size.

If Indices are not corrupted then after clearing cache and restarting the service, your Topology should show up just fine with all green tick marks under all the components.