Feature: Option to only use VSS

[netzwerg]

Hello,
i've tested with FFS a bit and it's a great piece of software.
I have one sugguestion which might be useful for those using FFS as a backup tool.
As far as I understand FFS currently uses VSS this way:
- normal sync from the original filesystem
- only when a file is locked VSS is used
- any other files are still from the original file systems.

I see two problems with this approach - two minor and a major one:
1. I noticed several times that FFS complains about missing files - files that were deleted between the time that FFS enumerated the files to sync and the time when it actually comes to syncing the files. Most times this doesn't matter much, but it's inconvenient and produces a lot of unneccessary errors/warnings.
2. VSS only comes into action if the file is already locked. If a process tries to open a file that FFS opened normally, lock will fail. For example Firefox will fall flat on its face when its files are locked.
3. Filesets that depend on each other will produce inconsistent backups. Examples for this are VMware VMs (2GB slices) or many tools that use BDB with logging - if you backup files that originate from differing points in time, backups will be inconsistent and - in the worst case - corrupt.

If FFS could use the VSS snapshot from start to end of the sync run all of these problems would vanish - because that's what VSS was made for: Consistent file system snapshots.

The only thing that changes from a user perspective: FFS will only sync files in the state they were at the start of the sync operation. In most cases this might be what the user actually wants and in the case of backups that is standard procedure.

What do you think?