How does RollBack Rx use disk space? Does Rollback 9.0 use dependents?

[Stu]

Using trial which will expire in 10days. So far, I'm very impressed but have a number of questions and issues. I will post one at a time as they are addressed before my trial expires.
First...
Are snapshots dependent on previous snapshots?
Scenario: Take a snapshot "A" which takes up 500mb, then install a couple of programs and take snapshot "B" which takes up 1000mb; then take another snapshot "C" immediately after snapshot "B" which takes up 0mb since nothing else was changed on computer since "B" was created.

Question: While in Snapshot "C", if I deleted Snapshot "B" and rollback to "A", and then from "A", rollback to "C" (0mb), will the programs I installed on "B" (deleted) still be present?

Thanks.
-stu

[Nick10]

Based on your example above - the answer is YES. The reason for this is that you have a snapshot "C" which was created based on snapshot "B". Since your applications that were installed on snapshot "B" also existed in snapshot "C" the files will still remain in snapshot "C".

I hope I didn't lose anyone on this...

[Nick10]

Originally Posted by Stu

Are snapshots dependent on previous snapshots?

___________

Here is an exerpt from our KB article on how RollBack Rx uses disk space which may help other users as well; to better understand RollBack's use of disk space:

Unlike conventional backup/recovery solutions, RollBack Rx protects the system and data without creating duplicated copies of the files. This concept, it is very often confusing to many users who are used to "conventional" backup/recovery solutions. The following questions and answers outlines the fundamental ideas of how RollBack Rx uses disk space.

How much disk space do I need to setup RollBack Rx?

RollBack Rx protects the system and data in real time without creating duplicate copies, it doesn't take or reserve a lot of disk space to setup. RollBack Rx takes about 0.07% of hard disk space plus another 10 or 20MB for help and other graphic files to setup.


For example

1. You have a 10GB hard drive with 5GB data and 5GB free space.
   
2. RollBack Rx setup would take about 20MB disk space (10000MB*0.07%=7 MB, the other 10-20MB for help files, graphics and etc).
   
3. After setup RollBack Rx, you would have about 4.98GB free space.

How much disk space do I need to take a snapshot?

The space taken by a new snapshot is amount of data that's added (changed) since last snapshot, which is also the size of the snapshot as shown in RollBack Rx Snapshot Management console.

For example

1. You install RollBack Rx, setup creates a baseline snapshot, and you have 5GB free space.
   
2. You install Microsoft Office 2003, it takes about 800MB. You have 4.2GB free space.
   
3. You take a snapshot "Office-snapshot", the size of this snapshot is 800MB. You have about 4.2GB free space. (Note RollBack Rx didn't take any additional disk space to take this snapshot)

How can I delete files and free disk space from snapshots?

To free disk space from snapshots, you must meet one condition: the files should not be contained in any other snapshots. The rule of thumb is that if a file is no where to be found, it shouldn’t take any disk space. But if it’s found in a snapshot, it will take disk space.

For example

1. You install RollBack Rx, setup creates a baseline snapshot, and you have 5GB free space.
   
2. You install Microsoft Office 2003, it takes about 800MB. You have 4.2GB free space.
   
3. You take a snapshot "Office-snapshot"; the size of this snapshot is 800MB. You have about 4.2GB free space.
   
4. You uninstall Office 2003, it removes all the office files. (The free space is still 4.2GB, because the office files are in “office-snapshot”. If you restore to the “office-snapshot”, Office 2003 will be there; therefore, it’s on the hard drive).
   
5. You take a snapshot "no-office"; the space taken by this snapshot is almost nothing, because no data has been added since last snapshot. You have about 4.2GB free space.
   
6. You delete snapshot "office-snapshot". At this point, we only have Baseline snapshot which does not have Office 2003 and "no-office" snapshot which does not have Office 2003. So the 800MB space used by Office 2003 should be freed.
   
7. Run snapshot defragmenter. The free space should be about 5GB.

I deleted a snapshot, ran snapshot defragmenter, but free space remains unchanged. What's wrong?

It's very likely the snapshot you just deleted has children snapshots which inherited the data.

For example

1. You install RollBack Rx, setup creates a baseline snapshot, and you have 5GB free space.
   
2. You install Microsoft Office 2003, it takes about 800MB.
   
3. You take snapshot "Office-snapshot", the size of this snapshot is 800MB. You have about 4.2GB free space.
   
4. You then take a new snapshot "new-snapshot"; "new-snapshot" is a child snapshot of "office-snapshot". There is no data added since "office-snapshot", so the size of "new-snapshot" is almost nothing. You have about 4.2GB free space.
   
5. You delete "office-snapshot", run snapshot defragmenter. But the free space is still at 4.2GB, no free space gained from deleting snapshot "office-snapshot". This is because the "new-snapshot" has the Office 2003 files, the space taken by Office 2003 cannot be freed.

[Stu]

Thanks for the reply, Nick. That's good news !

[Dallion]

In Stu's example snapshot C will report being "0MB" even after deleting snapshot B.

Hmmm... so why don't we always take two snapshots and immediately delete the first one thus saving us loads of disk space?

Well, it's really a technical illusion. So just to clarify for those who might think they can get "something for nothing" out of this...

The illusion is caused because the space taken by snapshot B is not reclaimed until the snapshots are defragged. Technically speaking, snapshot C isn't taking any room because the details of snapshot B still exist even though it's been deleted. But in reality, snapshot C is taking up 1000MB !
How can I prove this? It becomes apparent when you defrag the snapshots and watch how long it takes to defrag snapshot C. It's taking up 0MB remember, so you would expect defragging it to be really quick, right? Wrong. Once defragging is complete the size of Snapshot C will change from 0MB to correctly report as being 1000MB.

So if you need accurate stats, always defrag your snapshots after deleting!

(Nick10 - I carried out a small test to verify this, but please correct me if I'm wrong about any of the above)