What's the diff between a diff and an inc?

Discussion in 'Acronis True Image Product Line' started by Calhob, Aug 6, 2008.

Thread Status:
Not open for further replies.
  1. Calhob

    Calhob Registered Member

    Joined:
    Jul 5, 2008
    Posts:
    13
    I've studied and reread the manual and read and reread the posts on this forum and still I don't understand the difference between a differential and an incremental backup.

    Maybe I'm a bit slow witted but will someone please explain just what is saved in each of these types?

    I THINK an incremental saves in each increment only what was added to the file since the last increment, so you need the original full backup and all of the increments to do a restore.

    I THINK a differential saves in each backup all changes since the original full backup, so you need only the full backup and the latest diff backup file. If that is so, why would anyone bother doing incremental backups, which take up so much more disk space?
     
  2. seekforever

    seekforever Registered Member

    Joined:
    Oct 31, 2005
    Posts:
    4,751
    You have it correct.

    Incrementals are faster to do but take longer to restore. Differentials are the opposite.

    If you want to be able to restore your system to any particular point in time (for which you have a backup) then you have keep all of the differentials or incrementals. The differentials would then take a fair bit of disk space since the changed files are duplicated in each suceeding differential.

    I don't know if the bug has been fixed but in older versions of TI you couldn't do a Validate of the archive unless all of the differentials were present.
     
  3. dougaross

    dougaross Registered Member

    Joined:
    Jul 4, 2008
    Posts:
    59
    I beleive in T11 this is still true UNLESS you use a Backup Location for your archives
     
  4. SloPoke

    SloPoke Registered Member

    Joined:
    Jan 6, 2007
    Posts:
    34
    Why would a series of incrementals take up more disk space than a series of differentials? Differentials only get larger and larger, whereas incrementals vary in size depending on what's changed since the last incremental.

    I run incrementals every 2 hours in the background throughout the day; doing that with differentials would consume all my backup space very quickly.
     
  5. Calhob

    Calhob Registered Member

    Joined:
    Jul 5, 2008
    Posts:
    13
    SloPoke, seems to me more space because you have to keep them ALL to do a restore. With differentials you only have to keep the original and the latest diff. Much less space, I figure.
     
  6. shieber

    shieber Registered Member

    Joined:
    Oct 27, 2004
    Posts:
    3,710
    Unless you want o be able to restore to a point between the full and last diff--todo that, you'd need to save the other diffs.

     
  7. Earthling

    Earthling Registered Member

    Joined:
    Nov 5, 2007
    Posts:
    604
    Location:
    UK
    This still isn't really clear. Using v11 in Vista I normally do a full about once a month, and diffs whenever I feel like it, but usually about weekly. When I do a diff I usually delete the previous one, but there's a suggestion here that this will not restore unless you are using Backup Locations, which I don't. I just use my own backup folder structure.

    Is any one able to give a definitive answer to this, or have I got to switch to XP, backup in Ghost to be sure I can recover, and then see if my latest diff restores?
     
  8. seekforever

    seekforever Registered Member

    Joined:
    Oct 31, 2005
    Posts:
    4,751
    The issue pertains to doing a Validate of the archive not a restore.

    Sounds like you haven't done the test which is highly recommended by many on this forum - put in a spare HD and then confirm you can indeed restore an archive just like you would if your HD went bad. This recommendation is not only for TI but for any backup software. If you don't know for certain it will restore you shouldn't have much confidence in the process.
     
  9. Earthling

    Earthling Registered Member

    Joined:
    Nov 5, 2007
    Posts:
    604
    Location:
    UK
    Well, I was extrapolating - it if won't validate it may not restore either. If it's only validation then I'm not too bothered, as I know my system backs up and restores from fulls + incs, but have not restored since switching to fulls + diffs.
     
  10. Earthling

    Earthling Registered Member

    Joined:
    Nov 5, 2007
    Posts:
    604
    Location:
    UK
    Can't argue with that so thought I'd better put it to the test. I can confirm that if you use differential backups, and do NOT use Backup Locations, and if you have deleted any intermediate diffs, then Validate fails.

    In my case, happily, despite the Validation failure, Restore still worked successfully. :)

    I did not test the procedure when using Backup Locations so cannot confirm whether Validate works or not in that situation.
     
    Last edited: Aug 8, 2008
  11. seekforever

    seekforever Registered Member

    Joined:
    Oct 31, 2005
    Posts:
    4,751
    Thanks for the update and confirming some of the theories in the thread.
     
  12. Calhob

    Calhob Registered Member

    Joined:
    Jul 5, 2008
    Posts:
    13
    In my first post at the start of this thread I said I thought that, if you used differential backups, you only needed to keep the original full backup and the latest differential. Others agreed. However, I've now tested this by changing the file extension of all the differentials except the latest (thereby effectively deleting them) and doing a restore. It didn't work and asked for the FIRST differential. I then renamed the first of the differentials back to its original extension - and it worked.

    This means that you must keep the original full backup, the first differential and the latest differential to be able to do a restore. This is not mentioned in the manual or anywhere else that I have seen. Maybe Acronis should make this clear to its many customers.
     
  13. Earthling

    Earthling Registered Member

    Joined:
    Nov 5, 2007
    Posts:
    604
    Location:
    UK
    Read my #10. That isn't what I found. I only needed the full and the last diff to restore.

    You don't say which version you are using. I'm on v11:8101
     
  14. Calhob

    Calhob Registered Member

    Joined:
    Jul 5, 2008
    Posts:
    13
    That's really strange. I've tried it twice and both times it stopped and asked for the first diff. I'm using v 11 8101, same as you. And using a backup location.
     
    Last edited: Aug 9, 2008
  15. Earthling

    Earthling Registered Member

    Joined:
    Nov 5, 2007
    Posts:
    604
    Location:
    UK
    The only difference then is that I don't use backup locations, just my own folder structure. You might want to to try that.
     
  16. MudCrab

    MudCrab Imaging Specialist

    Joined:
    Nov 3, 2006
    Posts:
    6,483
    Location:
    California
    Calhob,

    Is the base Full Image contained all in one file or is the second file actually part of the first image and not a Differential? You should be able to tell from the time/date of the files.
     
  17. Calhob

    Calhob Registered Member

    Joined:
    Jul 5, 2008
    Posts:
    13
    It's a separate file and a complete full backup. The diff was created two days later. Both files are in an Acronis backup location on a dedicated HD partition. They are actually backups of my My Documents folder.
     
  18. Earthling

    Earthling Registered Member

    Joined:
    Nov 5, 2007
    Posts:
    604
    Location:
    UK
    Slightly off topic, but ATI is a real clumsy tool for backing up folders, and also has the disadvantage of putting your files into a proprietary format. Folder synchronisation programs such as SyncBack or M$' SyncToy beta are way superior, simple and fast to use, and everything kept in native format. SyncToy, despite its beta status, is a dream. If you need standard file compression then WinRAR is a good choice, but has a relatively weak UI.

    Just a thought.
     
  19. Tony Gamble

    Tony Gamble Registered Member

    Joined:
    Aug 8, 2008
    Posts:
    15
    So what is the conclusion for getting validate to work, please?

    I'm on Version 11 Home.

    Will it work if one leaves the first differential and the last differential undeleted - plus naturally the original?

    I accept that users above have found you can have the original and the final differential and the restore is OK. But I am a nervous person and cannot imagine that Acronis did not intend the validation to work.

    Tony
    London UK
     
  20. Calhob

    Calhob Registered Member

    Joined:
    Jul 5, 2008
    Posts:
    13
    Tony

    My experience is that a validation works fine if you use an Acronis Backup Location for your files. You validate each file separately so it doesn't matter how many you have.

    Also, in my experience, if you have the first and last differentials and the original, a restore works every time.

    Calhob
     
  21. Tony Gamble

    Tony Gamble Registered Member

    Joined:
    Aug 8, 2008
    Posts:
    15
    Thanks,

    I did not have an Acronis Backup Location. I simply used a self-created folder.

    I have created an ABL and done a full backup. I've scheduled my first differential and will see how that verifies.

    Thanks again.

    Tony
     
  22. shieber

    shieber Registered Member

    Joined:
    Oct 27, 2004
    Posts:
    3,710
    There's a known issue in ATI, going several versions back where an ATI validation on Diffs only works if all the intermediate diffs are present. Apparently, to fix this requires getting down pretty deep into some of the fundamental code and that's why Acronis has gotten around to it yet. Despite the validation issue, intermediate diffs are not needed to do a restore.

    Btw, an inc is a collection of changes since the last backup in the set, whether that last file was the full or a subsequent inc. You need all the intermediate incs to do a restore. A diff is a collection of changes since the last full in the set and intermediates are not needed to do a restore.

    Further, btw, Validation is a test of whether the file has changed since it was created -- it reads checksums that ATI embedded in the backup file upon creation and compares those to recomputed checksums made from the file data during the "Validation" test.
     
  23. Tony Gamble

    Tony Gamble Registered Member

    Joined:
    Aug 8, 2008
    Posts:
    15
    I've just noticed that having created a Backup Location on my USB remote drive I see a reference to an Acronis Secure Zone when I do my Schedule Task Wizard.

    From other threads I read that creating an ASZ on a separate USB hard drive usually creates more problems than it solves - so I have so far avoided creating one.

    As long as I put the backup in what is called Backup Locations am I avoiding the problems of ASZ or drifting into them?

    (Incidentally before anyone suggests I should read the V 11 manual I'll mention that the download site is not working at the mo'!)

    Kind regards,

    Tony
     
Thread Status:
Not open for further replies.