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@@ -8,125 +8,6 @@ Frequently asked questions
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Usage & Limitations
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###################
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-Can I backup VM disk images?
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-----------------------------
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-
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-Yes, the `deduplication`_ technique used by
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-Borg makes sure only the modified parts of the file are stored.
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-Also, we have optional simple sparse file support for extract.
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-
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-If you use non-snapshotting backup tools like Borg to back up virtual machines,
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-then the VMs should be turned off for the duration of the backup. Backing up live VMs can (and will)
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-result in corrupted or inconsistent backup contents: a VM image is just a regular file to
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-Borg with the same issues as regular files when it comes to concurrent reading and writing from
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-the same file.
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-
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-For backing up live VMs use file system snapshots on the VM host, which establishes
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-crash-consistency for the VM images. This means that with most file systems
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-(that are journaling) the FS will always be fine in the backup (but may need a
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-journal 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
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-like PostgreSQL, SQLite etc. but also for example Borg repositories) should always
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-be able to recover to a consistent state from a backup created with
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-crash-consistent snapshots (even on ext4 with data=writeback or XFS).
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-
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-Hypervisor snapshots capturing most of the VM's state can also be used for backups
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-and can be a better alternative to pure file system based snapshots of the VM's disk,
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-since no state is lost. Depending on the application this can be the easiest and most
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-reliable way to create application-consistent backups.
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-
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-Other applications may require a lot of work to reach application-consistency:
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-It's a broad and complex issue that cannot be explained in entirety here.
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-
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-Borg doesn't intend to address these issues due to their huge complexity
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-and platform/software dependency. Combining Borg with the mechanisms provided
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-by the platform (snapshots, hypervisor features) will be the best approach
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-to start tackling them.
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-
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-How can I decrease the size of disk image backups?
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---------------------------------------------------
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-
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-Full 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. This is because virtually all file systems
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-don't actually delete the data on disk (that is the place of so-called "secure delete")
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-but instead delete the filesystem entries referring to the data. This leaves the random
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-data on disk until the FS eventually claims it for another file. Therefore, if a hard
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-drive nears capacity and files are deleted again, the change will barely decrease the
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-space it takes up when compressed and deduplicated. Depending on the filesystem of the
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-VM (or physical computer, if for some reason a normal filesystem backup can't be taken),
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-there are several ways to decrease the size of a full 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,
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-when it can be used, its special image format is even more efficient than just zeroing
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-and 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 $x
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- done
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-
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-Restoration is similar to the above process, but done in reverse::
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-
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- borg extract --stdout repo::hostname-partinfo | dd of=$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, except
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-that it works in place, zeroing the original partition. This makes the backup process a bit
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-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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Can I backup from multiple servers into a single repository?
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------------------------------------------------------------
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Milkey Mouse