Storage expansion for a virtual machine operating system is reduced. In one embodiment, virtual machines are run on a host and accessed by remote clients over a network. When a guest operating system on one of the virtual machines deletes a file, a VM storage manager on the host detects a special write performed by the guest operating system that writes zeros into a logical block of the file. The VM storage manager links the logical block to a designated block, and de-allocates the disk block that is mapped to the logical block. The de-allocation allows the disk block to be reused by the virtual machines.