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+.. include:: ../global.rst.inc
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+.. highlight:: none
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+
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+Backing up entire disk images
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+=============================
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+
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+Backing up disk images can still be efficient with Borg because its `deduplication`_
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+technique makes sure only the modified parts of the file are stored. Borg also has
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+optional simple sparse file support for extract.
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+
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+Decreasing the size of image backups
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+------------------------------------
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+
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+Disk images are as large as the full disk when uncompressed and might not get much
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+smaller post-deduplication after heavy use because virtually all file systems don't
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+actually delete file data on disk but instead delete the filesystem entries referencing
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+the data. Therefore, if a disk nears capacity and files are deleted again, the change
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+will barely decrease the space it takes up when compressed and deduplicated. Depending
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+on the filesystem, there are several ways to decrease the size of a disk image:
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+
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+Using ntfsclone (NTFS, i.e. Windows VMs)
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+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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+
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+``ntfsclone`` can only operate on filesystems with the journal cleared (i.e. turned-off
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+machines), which somewhat limits its utility in the case of VM snapshots. However, when
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+it can be used, its special image format is even more efficient than just zeroing and
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+deduplicating. For backup, save the disk header and the contents of each partition::
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+
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+ HEADER_SIZE=$(sfdisk -lo Start $DISK | grep -A1 -P 'Start$' | tail -n1 | xargs echo)
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+ PARTITIONS=$(sfdisk -lo Device,Type $DISK | sed -e '1,/Device\s*Type/d')
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+ dd if=$DISK count=$HEADER_SIZE | borg create repo::hostname-partinfo -
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+ echo "$PARTITIONS" | grep NTFS | cut -d' ' -f1 | while read x; do
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+ PARTNUM=$(echo $x | grep -Eo "[0-9]+$")
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+ ntfsclone -so - $x | borg create repo::hostname-part$PARTNUM -
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+ done
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+ # to backup non-NTFS partitions as well:
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+ echo "$PARTITIONS" | grep -v NTFS | cut -d' ' -f1 | while read x; do
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+ PARTNUM=$(echo $x | grep -Eo "[0-9]+$")
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+ borg create --read-special repo::hostname-part$PARTNUM $x
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+ done
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+
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+Restoration is a similar process::
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+
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+ borg extract --stdout repo::hostname-partinfo | dd of=$DISK && partprobe
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+ PARTITIONS=$(sfdisk -lo Device,Type $DISK | sed -e '1,/Device\s*Type/d')
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+ borg list --format {archive}{NL} repo | grep 'part[0-9]*$' | while read x; do
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+ PARTNUM=$(echo $x | grep -Eo "[0-9]+$")
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+ PARTITION=$(echo "$PARTITIONS" | grep -E "$DISKp?$PARTNUM" | head -n1)
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+ if echo "$PARTITION" | cut -d' ' -f2- | grep -q NTFS; then
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+ borg extract --stdout repo::$x | ntfsclone -rO $(echo "$PARTITION" | cut -d' ' -f1) -
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+ else
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+ borg extract --stdout repo::$x | dd of=$(echo "$PARTITION" | cut -d' ' -f1)
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+ fi
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+ done
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+
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+.. note::
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+
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+ When backing up a disk image (as opposed to a real block device), mount it as
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+ a loopback image to use the above snippets::
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+
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+ DISK=$(losetup -Pf --show /path/to/disk/image)
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+ # do backup as shown above
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+ losetup -d $DISK
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+
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+Using zerofree (ext2, ext3, ext4)
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+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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+
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+``zerofree`` works similarly to ntfsclone in that it zeros out unused chunks of the FS,
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+except it works in place, zeroing the original partition. This makes the backup process
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+a bit simpler::
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+
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+ sfdisk -lo Device,Type $DISK | sed -e '1,/Device\s*Type/d' | grep Linux | cut -d' ' -f1 | xargs -n1 zerofree
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+ borg create --read-special repo::hostname-disk $DISK
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+
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+Because the partitions were zeroed in place, restoration is only one command::
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+
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+ borg extract --stdout repo::hostname-disk | dd of=$DISK
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+
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+.. note:: The "traditional" way to zero out space on a partition, especially one already
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+ mounted, is to simply ``dd`` from ``/dev/zero`` to a temporary file and delete
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+ it. This is ill-advised for the reasons mentioned in the ``zerofree`` man page:
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+
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+ - it is slow
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+ - it makes the disk image (temporarily) grow to its maximal extent
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+ - it (temporarily) uses all free space on the disk, so other concurrent write actions may fail.
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+
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+Virtual machines
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+----------------
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+
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+If you use non-snapshotting backup tools like Borg to back up virtual machines, then
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+the VMs should be turned off for the duration of the backup. Backing up live VMs can
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+(and will) result in corrupted or inconsistent backup contents: a VM image is just a
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+regular file to Borg with the same issues as regular files when it comes to concurrent
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+reading and writing from the same file.
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+
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+For backing up live VMs use filesystem snapshots on the VM host, which establishes
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+crash-consistency for the VM images. This means that with most file systems (that
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+are journaling) the FS will always be fine in the backup (but may need a journal
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+replay to become accessible).
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+
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+Usually this does not mean that file *contents* on the VM are consistent, since file
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+contents are normally not journaled. Notable exceptions are ext4 in data=journal mode,
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+ZFS and btrfs (unless nodatacow is used).
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+
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+Applications designed with crash-consistency in mind (most relational databases like
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+PostgreSQL, SQLite etc. but also for example Borg repositories) should always be able
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+to recover to a consistent state from a backup created with crash-consistent snapshots
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+(even on ext4 with data=writeback or XFS). Other applications may require a lot of work
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+to reach application-consistency; it's a broad and complex issue that cannot be explained
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+in entirety here.
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+
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+Hypervisor snapshots capturing most of the VM's state can also be used for backups and
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+can be a better alternative to pure file system based snapshots of the VM's disk, since
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+no state is lost. Depending on the application this can be the easiest and most reliable
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+way to create application-consistent backups.
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+
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+Borg doesn't intend to address these issues due to their huge complexity and
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+platform/software dependency. Combining Borg with the mechanisms provided by the platform
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+(snapshots, hypervisor features) will be the best approach to start tackling them.
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TW