Volume and Snapshot Usage

Space used by a snapshot is never more than the space used by the volume at the time the snapshot was created. Any "live" new data introduced by snapshots is attributed to the live volume space usage and not the snapshot usage. Blocks are attributed to snapshot space usage when they are overwritten in live state. The easiest way to think about this is that snapshots consume space only when a block is modified or deleted.

For example, if you write 50 GB to a volume and take a snapshot, the initial snapshot shows zero snapshot usage, because it points to the same blocks on disk, which are "live." If you write another 50 GB to the volume, making a total of 100 GB, and then take another snapshot, the snapshots still consume no space. In fact, you can create several snapshots and they will all consume no additional space for the newly written 100 GB.

If you overwrite the 100 GB, then all snapshots start to consume space. Their cumulative space usage should be approximately 100 GB.

When some applications, such as PowerPoint, Excel, and Word modify files, the applications do not necessarily update the specific blocks in the file that changed. Instead, the applications create a new file. In this case, after you modify a file from one of these applications, a snapshot could show no usage because the original file the snapshot points to was not changed, but instead was seen as a new file.