1. A system for tracing states of Virtual Execution Environments (VEEs) without freezing service operations, 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 on the computer system under the host OS, wherein the VEEs provide services to a plurality of remote users, and wherein each VEE has a VEE file system; at least one designated VEE providing tracing services to other VEEs of the plurality of VEEs; and a tracing application running on a Guest OS of the designated VEE, wherein the tracing application generates consistent snapshots of the VEEs' file system by (a) creating a bitmap of a snapshot of the target VEE file system and simultaneously tracking changes of files of the target VEE file system, (b) recording the changes of the files, (c) freezing write operations to the target VEE file system, (d) updating the bitmap of the snapshot according to recorded file changes, (e) generating the snapshot of the target VEE files system based on the bitmap of the file system snapshot, and (f) releasing the write operations to the target VEE file system.