Differential vs. incremental ...

Discussion in 'Acronis True Image Product Line' started by msanto, Sep 21, 2005.

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  1. msanto

    msanto Registered Member

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    what's the difference. To me it sounds like both of them are just the changes from the last full backup. I must be missing something.
     
  2. MerlinAZ

    MerlinAZ Registered Member

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    My understanding is this.
    Incremental compares everything since the last backup. If you only have a full backup, it compares itself to that. If you have any Incremental backups, the new Incremental compares itself to the last Incremental. In order to restore, you will need the original full backup and all incremental backups after.

    Differential backups always compare themselves to the original full backup only. So if you have multiple differential backups, the restore will only need the latest differential backup and the original full backup.
    I don't think it needs the other differential backups if there are any.

    Like I said, this is my understanding open to any corrections from posters below me.
     
    Last edited: Sep 22, 2005
  3. Menorcaman

    Menorcaman Retired Moderator

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    Hi MerlinAZ,



    More or less correct, although it would have been more accurate if you'd said "You will need the original full backup and all the incrementals up to the point in time you selected to restore".

    I think you meant say ".....latest differential...." rather than latest incremental ;). Also, as before, it's more accurate to say that you only need the original full backup and the differential equating to the point in time you selected to restore.

    The graphic provided by bobat in this previous thread titled <Incremental Backup> illustrates the difference between Full, Incremental and Differential images quite well.

    Regards
     
  4. Acronis Support

    Acronis Support Acronis Support Staff

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    Hello msanto,

    Thank you for your interest in Acronis Disk Backup Software.

    Differential backup contains all data modified since any preceding backup (unlike incremental backup that contains data from the last backup only). To restore from a differential backup, one needs to have all backups preceding the backup being restored.

    Thank you.
    --
    Irina Shirokova
     
  5. MerlinAZ

    MerlinAZ Registered Member

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    Thanks Menorcaman, I edited my post. ;)
    I think Irina confused the two since to restore from a differential you don't need all prior backups, just the latest differential (from the point in time you want) and latest full backup, not all the other differentials.
     
  6. Acronis Support

    Acronis Support Acronis Support Staff

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    Hello MerlinAZ,

    You are absolutely correct to restore a differential backup you should have the latest differential and the latest full backups.

    Thank you.
    --
    Irina Shirokova
     
  7. crofttk

    crofttk Registered Member

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    :eek:
    Ummmmm.... I think that one just threw the whole explanation back into a quagmire of ambiguity !

    How about:

    "ASSUMING THERE HAVE BEEN NO OTHER INTERVENING BACKUPS OTHER THAN ANOTHER DIFFERENTIAL BACKUP, Differential backup contains all data modified since any preceding FULL backup (unlike incremental backup that contains data from CHANGED SINCE the last FULL OR INCREMENTALbackup only). To restore from a differential backup, one needs to have all FULL AND INCREMENTALbackups preceding the backup being restored." ?

    But that's rather butchered, so, unless Acronis has deviated from the long pre-established definitions of full, incremental, and differential backups, I would say:

    1) Each file has an "archive" bit as part of it's description. Any time a file is modified or newly created, that file's archive bit is GENERALLY "set" (i.e., switched from a value of 0 to a value of 1). This serves as an indicator that a backup of this file or version of the file doesn't exist.

    2) Any time a file is either included in either a FULL or INCREMENTAL backup, the file's archive bit is "cleared" (i.e., switched from a value of 1 back to a value of 0).

    3) The relevant EXCEPTION to 1) and 2) is that a DIFFERENTIAL backup will NOT clear the archive bit (so that the file is again caught in the next backup sweep).

    So:

    IF one uses a FULL plus INCREMENTAL backup "strategy",

    THEN, to restore everything to the most recent backup point, one must restore the original FULL backup plus ALL intervening INCREMENTAL backups.


    BUT

    IF one uses a FULL plus DIFFERENTIAL backup "strategy",

    THEN, to restore everything to the most recent backup point, one need only restore the original FULL backup plus THE MOST RECENT DIFFERENTIAL backup.


    I hope that is slightly clearer than mud.

    HOWEVER, I do see that both MerlinAZ and Irina overcame that obstacle while I labored to create a less ambiguous explanation of my own !
     
    Last edited: Sep 22, 2005
  8. crofttk

    crofttk Registered Member

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    Hmmm...

    I see from another post that ATI actually uses a sector by sector comparison to pick up file changes and will base incrementals or differentials on even a movement of a file from one physical location to another. It's not as simple (or complex ?) as relying on the archive bit !

    I read this some time ago and should have know better. :rolleyes:

    While apologizing for my inaccuracy, I will stand by my attempt to explain how incrementals and differentials have traditionally differed when based on the archive bit.
     
  9. msanto

    msanto Registered Member

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    OK, so here's my attempt to sort through all this.

    Let's say you do a FULL Backup on 9/1/2005. If you do an INCREMENTAL on 10/1 and another on 11/1, you will need: the FULL, and BOTH the 10/1 AND the 11/1 INCREMENTALS to restore.

    If you do a DIFFERENTIAL on 10/1 and another on 11/1, you will ONLY need the FULL and the DIFFERENTIAL on 11/1 (you can basically delete the 10/1) to restore.

    Did I get it right?
     
  10. crofttk

    crofttk Registered Member

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    As far as I'm concerned, you did.
     
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