Space reclamation is a background process. This normally is a slow process that will greatly consume resource on the filer. It would be recommended to perform space reclamation during the night time when there are less jobs running on the filer. As the Volumes are spanned within the Raid Groups of the Aggregates it will take WALF some time to free the blocks.
SnapDrive space reclamation in Windows Servers – Frequently Asked Question (FAQ)
Space Reclamation: If Being Thin is Attractive, Then Thin Provisioning Makes for Really Sexy Storage
NetApp Thin-Provisioned LUNs on RHEL 6.2 Deployment Guide that outlines space reclamation best practices on Red Hat Linux