卷影复制（VSS）

Question

Can anyone clarify an issue? I'm using the VSS API (C++ using VSS2008 and the latest SDK running on XP SP3) in a home-brew backup utility*.

THe VSS snapshot operations work fine for folder that have no subfolders - ie my email and SQL server volumes. However when I take a snapshot of a folder that does contain subfolders, the nested structure is 'flattened' in the snapshot - all sub-directories cease to exist.

So here is the question: I am aware that support for VSS on XP is a bit limited but is there a way to specify a snapshot be non-recursive? The docs are not very helpful ...

• I got really tired of buggy rubbish that costs boatloads and fails every few days so I thought I'd roll my own. It'll get onto CodeProject at some point. If anyone is interested let me know and you can have a (source) copy when it's ready ...

Thx++

Jerry

Answer 1

Your question is confusing...

VSS does not work at a "folder" level. It works a "volume" level.

You "snap" a volume and you will have a device path which you can "open" using the filesystem api (which will automatically mount the device volume with a filesystem) on a file by file or you can access the device directly (sector by sector).

It should be easy to backup all files on the snapped device volume (don't forget all of the file streams and ACL's for NTFS files), your problem will be restoring them... VSS will not help you on the restore. The main problem will be restoring a system volume, where you will need another OS to boot to like WinPE or DOS or something else. If your not worried about system volumes then restore can be easy.

If you backup the data in terms of sectors, then you get the added benefit that if you write a volume device driver for it (to look like a volume or HD) then windows will auto-load a filesystem driver for it. This gives you a free explorer application, this is what most sector based backup applications do. Also it gives them VM possibilities.

Even if you are doing simple file backups, it helps to understand filesystems (NTFS, FAT, etc) so that you know what you can/should backup and restore. Do you know what a NTFS reparse point is? How are you going to deal with it if you hit one during your backup? Do you know how windows actually boots and what files you need to backup and restore and "patch" to be able to have a chance at booting. On a restore, how best do you lay out the NTFS volume as not to affect NTFS performance on the restored volume? Are you going to support restoring system volumes to new hardware, what does that require you to do just to have a chance of working? The questions are endless.

System backup/restore is not easy, there are lots of edge cases (see some of the questions above) that you don't know about until you hit them.

Good luck on you project, I hope I haven't put you off too much, I'm just saying there is a lot of work to be able to deliver a backup application that most people have have no idea about.

Answer 2

Comment on the above - if a 'writer' is playing the VSS game then it will ensure that the file system is in a happy state as part of the VSS setup.

In the case of MS SQL Server - check that it is a VSS writer. If it is then your snapshot of the DB files should be OK. If not, then its in what is called a 'crash state'. So for example if you are using MySQL or some other non-MS, non-VSS aware SQL database - your backup may or may not be coherent ('a good one'). In that case it may be better than nothing, but it it may also still be useless. Using VSS MAY result in a better integrity from which to make your backup, but of the files are open, they are open and if the app does not play in the VSS pig-pen then you may or may not be hosed.