Differential Backup Problems
I'm using a trial version of True IMage 2011, and was doing some basic testing to see how it works. I seem to be running into a problem with how differential backups are created, and am hoping someone can help me out.
Here's my test. I created a folder under My Documents (Backup Test folder) and created two text files. I ran a full backup selecting only this folder, and as expected, both files were included. I then ran a differential on the same folder, and since I hadn't touched either file, I expected neither to be included. To my surprise, both were included on the differential backup. I then edited one of the files, ran the differential again, and both were still there; I expected only the one edited version to be there, since the other was unchanged. I added another file, this time a Word file, and performed similar tests with similar results; in each case, all files were included.
I think I understand the concept of differential backups, so I'm confused about why it's still backing up all files. This is important to me, since the files I'm backing up (My Documents) exceed the capacity of some of my external hard drives and I'm planning to work a differential backup into my routine to allow me to backup more often and using smaller drives.
Any tips would be appreciated.
Thanks.
Yes, I opened the backup file (I used Zip format) to see which files were present. The fact that they are present tells me they were backed up, right?
Maybe I have a conceptual problem. I understood that the differential backup would only include those files that have changed since the last full backup. I had assumed that this was based on an archive bit for each file. Is this how it works? If so, what am I doing wrong based on the steps I outlined above?
In my answer I assumed that you back up to .tib format. I don't know how it works with differentials in case of .zip. ( it is supposed to according to userguide - ...gives the zip format most of the functionality available for the tib format. You can schedule backups, ... make incremental and differential backups, etc.)
Todd, I also am a new user of Acronis TI (Trial Version)
Regarding your Post:
I also had to understand the difference between "differential" and "incremental" backups.
If I am performaing differential backups then the basis is the last "full" backup.
Files changed since then will be backed-up.
If I do an "incremental" backup then the basis is the LAST backup.
Files changed since then will be backed-up.
The issue is something called the "archive bit" of the file.
This bit is a part of the files' attributes.
Some files have hidden attributes and some have system attributes.
Some files can have a combination of both (plus others).
These attributes are shown when going into Windows Explorer and right-clicking on a file to see its properties.
You see the Read-only and Hidden check boxes and an "Advanced" button (which you can click).
Clicking the Advanced button will show a check box (among others) which says "This file is ready for archiving".
And here lies the issue...
When a file is modified (changed) then the "archive bit" is set on.
The archive bit is a flag: this file has changed.
A primary reason for the archive bit is backups.
If I do differential backups then the archive bit IS NOT TURNED-OFF UNTIL the next "full" backup.
In other words, the file will still show ready for archiving.
The "full" backup is the basis.
Only files that have changed since the LAST FULL BACKUP.
If I do incremental backups then the archive bit IS TURNED-OFF WHEN the file is included in a backup.
Thus, only files that have changed since the LAST backup.
Hope this helps...
An afterthought re: Differential Backup...
The key to Differential -vs- Incremental is NOT the backup....it is the Restore/Recovery (R/R).
If I want to R/R using Differential then I only need the LAST FULL and the LAST DIFFERENTIAL.
If I want to R/R using Incremental then I need the LAST FULL and ALL OF THE INCREMENTAL's.
ATI doesn't use the archive bit. It calculates a hash value for the file to identify whether there has been a change.
A differential backup will contain all the changes since the last full backup.
I am not sure what procedure you went through. If you run a full backup and then a differential, ATI will start the differential as a new full backup.
Do the test the following way. Create a new backup task as a differential backup. Run it once. It will be a full backup. Run it a second time (the same task), it will be a partial backup.