I found the following:
http://www.microsoft.com/resources/...erv/2003/all/techref/en-us/w2k3tr_frs_how.asp
FRS Interfaces
Windows Server 2003 includes an FRS writer for the Volume Shadow Copy
service. The FRS writer allows Volume Shadow Copy service-compatible backup
programs, such as Windows Backup, to make point-in-time, consistent backups
of the replica tree. Using a Volume Shadow Copy service-compatible backup
program ensures that replica tree backups do not contain partial files
(which can occur when a replica member receives an updated file and begins
overwriting the previous version of the file in the replica tree with the
updated version of the file). This process, known as the install process,
creates a window during which the file is half-written. The window exists
because FRS must move the updated file from the preinstall folder and
overwrite the existing file in the replica tree.
Before a Volume Shadow Copy service-compatible backup program takes a shadow
copy of a replica tree, the program instructs FRS to stop requesting new
work items from the install command server queue. After all currently active
installations are complete, FRS enters a frozen state during which no
installations occur. Shadow copies made while FRS is in a frozen state will
not contain partial files. The install service thaws either when the backup
program signals a thaw or at a predetermined time-out period.
