What is VSS (Volume Shadow Copy Service)

Question

I read about VSS here. According to the article , VSS takes a backup of the entire volume. Where does this backup of the Volume get stored ? If its on the volume itself then its pretty unsafe right ?

Does it work like Hardware Snaps ?

Answer 1

To maintain efficiency, shadow copies are indeed stored on the same volume. That’s just how snapshots work. It’s the same with ZFS.

As for the how:

The copy-on-write method creates shadow copies that are differential rather than full copies of the original data. Like the clone method of creating shadow copies, the copy-on-write method can produce shadow copies using either software or hardware solutions. This method makes a copy of the original data before it is overwritten with new changes, as shown in the following table. When a change to the original volume occurs, but before it is written to disk, the block about to be modified is read and then written to a “differences area”, which preserves a copy of the data block before it is overwritten with the change. Using the blocks in the differences area and unchanged blocks in the original volume, a shadow copy can be logically constructed that represents the shadow copy at the point in time in which it was created.

(Emphasis mine)

This means that initially, a shadow copy is just a flag. As data starts to change, changed data is backed up to different locations on the disk so the original data stays available.

Answer 2

VSS snapshots are stored under the folder System Volume Information.

The folder contains information that casual interference could cause problems with proper system functioning. Here are some of the things kept in that folder.

• Information used by the Volume Snapshot Service (also known as "Volume Shadow Copy") so you can back up files on a live system.