Backing up entire disk images

Backing up disk images can still be efficient with Borg because its deduplication technique makes sure only the modified parts of the file are stored. Borg also has optional simple sparse file support for extract.

Decreasing the size of image backups

Disk images are as large as the full disk when uncompressed and might not get much smaller post-deduplication after heavy use because virtually all file systems don’t actually delete file data on disk but instead delete the filesystem entries referencing the data. Therefore, if a disk nears capacity and files are deleted again, the change will barely decrease the space it takes up when compressed and deduplicated. Depending on the filesystem, there are several ways to decrease the size of a disk image:

Using ntfsclone (NTFS, i.e. Windows VMs)

ntfsclone can only operate on filesystems with the journal cleared (i.e. turned-off machines), which somewhat limits its utility in the case of VM snapshots. However, when it can be used, its special image format is even more efficient than just zeroing and deduplicating. For backup, save the disk header and the contents of each partition:

HEADER_SIZE=$(sfdisk -lo Start $DISK | grep -A1 -P 'Start$' | tail -n1 | xargs echo)
PARTITIONS=$(sfdisk -lo Device,Type $DISK | sed -e '1,/Device\s*Type/d')
dd if=$DISK count=$HEADER_SIZE | borg create repo::hostname-partinfo -
echo "$PARTITIONS" | grep NTFS | cut -d' ' -f1 | while read x; do
    PARTNUM=$(echo $x | grep -Eo "[0-9]+$")
    ntfsclone -so - $x | borg create repo::hostname-part$PARTNUM -
# to backup non-NTFS partitions as well:
echo "$PARTITIONS" | grep -v NTFS | cut -d' ' -f1 | while read x; do
    PARTNUM=$(echo $x | grep -Eo "[0-9]+$")
    borg create --read-special repo::hostname-part$PARTNUM $x

Restoration is a similar process:

borg extract --stdout repo::hostname-partinfo | dd of=$DISK && partprobe
PARTITIONS=$(sfdisk -lo Device,Type $DISK | sed -e '1,/Device\s*Type/d')
borg list --format {archive}{NL} repo | grep 'part[0-9]*$' | while read x; do
    PARTNUM=$(echo $x | grep -Eo "[0-9]+$")
    PARTITION=$(echo "$PARTITIONS" | grep -E "$DISKp?$PARTNUM" | head -n1)
    if echo "$PARTITION" | cut -d' ' -f2- | grep -q NTFS; then
        borg extract --stdout repo::$x | ntfsclone -rO $(echo "$PARTITION" | cut -d' ' -f1) -
        borg extract --stdout repo::$x | dd of=$(echo "$PARTITION" | cut -d' ' -f1)


When backing up a disk image (as opposed to a real block device), mount it as a loopback image to use the above snippets:

DISK=$(losetup -Pf --show /path/to/disk/image)
# do backup as shown above
losetup -d $DISK

Using zerofree (ext2, ext3, ext4)

zerofree works similarly to ntfsclone in that it zeros out unused chunks of the FS, except it works in place, zeroing the original partition. This makes the backup process a bit simpler:

sfdisk -lo Device,Type $DISK | sed -e '1,/Device\s*Type/d' | grep Linux | cut -d' ' -f1 | xargs -n1 zerofree
borg create --read-special repo::hostname-disk $DISK

Because the partitions were zeroed in place, restoration is only one command:

borg extract --stdout repo::hostname-disk | dd of=$DISK


The “traditional” way to zero out space on a partition, especially one already mounted, is to simply dd from /dev/zero to a temporary file and delete it. This is ill-advised for the reasons mentioned in the zerofree man page:

  • it is slow
  • it makes the disk image (temporarily) grow to its maximal extent
  • it (temporarily) uses all free space on the disk, so other concurrent write actions may fail.

Virtual machines

If you use non-snapshotting backup tools like Borg to back up virtual machines, then the VMs should be turned off for the duration of the backup. Backing up live VMs can (and will) result in corrupted or inconsistent backup contents: a VM image is just a regular file to Borg with the same issues as regular files when it comes to concurrent reading and writing from the same file.

For backing up live VMs use filesystem snapshots on the VM host, which establishes crash-consistency for the VM images. This means that with most file systems (that are journaling) the FS will always be fine in the backup (but may need a journal replay to become accessible).

Usually this does not mean that file contents on the VM are consistent, since file contents are normally not journaled. Notable exceptions are ext4 in data=journal mode, ZFS and btrfs (unless nodatacow is used).

Applications designed with crash-consistency in mind (most relational databases like PostgreSQL, SQLite etc. but also for example Borg repositories) should always be able to recover to a consistent state from a backup created with crash-consistent snapshots (even on ext4 with data=writeback or XFS). Other applications may require a lot of work to reach application-consistency; it’s a broad and complex issue that cannot be explained in entirety here.

Hypervisor snapshots capturing most of the VM’s state can also be used for backups and can be a better alternative to pure file system based snapshots of the VM’s disk, since no state is lost. Depending on the application this can be the easiest and most reliable way to create application-consistent backups.

Borg doesn’t intend to address these issues due to their huge complexity and platform/software dependency. Combining Borg with the mechanisms provided by the platform (snapshots, hypervisor features) will be the best approach to start tackling them.