files quickly by setting the valid data length on files without force-writing data with zeroes up to the VDL (SetFileValidData function). For instance, this function can be used to quickly create a fixed size virtual machine hard disk. The default access control lists for newly created files are read-only for the Users group and write permissions are given only to the Administrators group, the System account and the owner. Faster boot and application launch The ability to boot in 30 seconds was a design goal for Windows XP, and Microsoft's developers made efforts to streamline the system as much as possible; The Logical Prefetcher is a significant part of this; it monitors what files are loaded during boot, optimizes the locations of these files on disk so that less time is spent waiting for the hard drive's heads to move and issues large asynchronous I/O requests that can be overlapped with device detection and initialization that occurs during boot. The prefetcher works by tracing frequently accessed paged data which is then used by the Task Scheduler to create a prefetch-instructions file at %WinDir%\Prefetch. Upon system boot or the launch of an application, any data and code in the trace that is not already in memory is prefetched from the disk. The previous prefetching results determine which scenario benefited more and what should be prefetched at the next boot or launch. The prefetcher also uses the same algorithms to reduce application startup times. To reduce disk seeking even further, the Disk Defragmenter is called in at idle time to optimize the layout of these specific files and metadata in a contiguous area. Boot and resume operations can be traced and analyzed using Bootvis.exe. Logon and logoff changes Windows XP includes a Fast Logon Optimization feature that performs logon asynchronously without waiting for the network to be fully initialized if roaming user profiles are not set up. Use of cached credentials avoids delays when logging on to a domain. Group Policy is applied in the background, and startup or logon scripts execute asynchronously by default. Windows XP reconciles local and roaming user profiles using a copy of the contents of the registry. The user is no longer made to wait as in Windows 2000 until the profile is unloaded. Windows XP saves locked registry hives with open keys after 60 seconds so that roaming profile changes can be saved back to the