1. A system for tracing states of Virtual Execution Environments (VEEs), the system comprising: a computer system including a processor and a memory; a host operating system (OS) running on the computer system; a plurality of VEEs running under the host OS, wherein the VEEs provide services to remote users, and wherein each VEE has a VEE file system; and a designated VEE running a tracing application that generates consistent snapshots by (a) creating a bitmap of a snapshot of a target VEE file system and tracking changes of files of the target VEE file system, (b) recording the changes of the files, (c) freezing writes to the target VEE file system, (d) updating the bitmap according to the changes of the files, (e) generating the snapshot of the target VEE files system based on the bitmap, and (f) releasing the writes to the target VEE file system.