Windows Vista I/O Technologies - Offline Files

Offline Files is a feature of Windows, introduced in Windows 2000, which maintains a client side cache of files shared over a network. It locally caches shared files marked for offline access, and uses the cached copy whenever the network connection to the remote files is interrupted. Windows Vista Business, Enterprise and Ultimate editions contain significant improvements to Offline Files. Beginning with Windows Vista, whenever the connection is restored, all open file handles to the cached copy are redirected to the remote version, without waiting for the cached files to be synchronized. This transition from online to offline and back is transparent to the clients using the file. The local copy is automatically synchronized with the remote copy, to reflect the changes made on either copy of the file. The file caching and sync algorithm has also been completely rewritten to determine the differences faster. When synchronizing the changes in the cached copy to the remote version, the Bitmap Differential Transfer protocol is used so that only the changed blocks in the cached version are transferred. This also improves support for caching large files. The entire file is still downloaded when retrieving changes from the remote copy.

Files are synchronized on a per-share basis and encrypted on a per-user basis and users can force Windows to work in offline mode or online mode through the Work Offline/Online button in Explorer, or sync manually from the Sync Center. Sync Center can also report sync errors and resolve sync conflicts. The property page for any file or folder has an Offline Files tab that provides status and allows control of the offline status of the file or folder. Moreover, even if a single file is unavailable, other files in the same share and other shares are available as the transition is now at the share level instead of server level. Offline Files are configurable through Group Policy and provide better interoperability with DFS. Also, a comprehensive Offline Files management API is available via COM objects and scriptability through WMI.

Windows Vista also supports "ghosting" of online files and folders. When users make only a few files from a directory available offline, Windows Vista creates ghosted entries of the remaining unavailable items to preserve the online context. Offline Files also feature slow-link mode which when enabled through Group Policy, always reads from and writes to the local cache to improve performance over a slow network connection. It is also possible in Windows Vista to specify a limit for the total size of the local cache and another sub-limit for the space used by automatically cached files. Manually cached files are never removed from the local cache even if the cache limit is reached.

In Windows XP, Offline Files could not be enabled when Fast User Switching was enabled. This restriction applied because Offline Files were synchronized at log off and Fast User Switching does not completely log off users. In Windows Vista, this restriction no longer applies as Offline Files runs as a Windows service that performs synchronization for the user at opportune times such as logon and offline to online transitions. Synchronization does not occur continuously in the background, nor does it occur at log off.