2712: Acronis Backup Software Creates Large Incremental or Differential Backup Archives

Incremental or differential backups are larger when performed after disk defragmentation or full restore

This article applies to all Acronis backup software.

Symptoms

  1. Using Acronis backup software you create an incremental or differential backup archive;
  2. The size of the created incremental or differential backup is almost the size of the full one.

Solution

There are several possible causes:

  • Defragmentation - An incremental or differential backup created after the disk is defragmented will be considerably larger than usual. This is because the defragmentation program changes file locations on the disk, and backups reflect these changes.
    (!) In Windows Vista and Windows 7 disk defragmentation is enabled by default. For information how to reschedule or disable it, see Microsoft Windows help page: Start Disk Defragmenter. After defragmenting a disk, create a new full backup of it.
  • Creating an incremental or differential backup after recovering the full one – An incremental or differential backup created after the full one was restored will be considerably larger than usual. This is because restoring a backup archive changes file locations on the disk, and backups reflect these changes. After restoring, create a new full backup of what you restored.
  • You have a full backup on the drive/location that you are backing up. It is not recommended to keep the backup on the same partition that you are backing up.
  • Applicable to Acronis Backup for VMware, Acronis Backup Advanced for VMware: Changed Block Tracking (CBT) VMware system improperly detects the changed sectors range on the backed up VM virtual disks.
    To find out whether the problem is CBT-related, create a backup with CBT option turned off (in backup settings); in this case Acronis Backup will calculate the changes on its own. If incrementals become normal, this means that CBT for the affected VMs has to be reset (disabling/re-enabling it) as described in Enabling Changed Block Tracking (CBT) on virtual machines. Note that these steps imply powering off the problematic VM.

More information

See also:

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