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Move image backup-related FAQ entries to a new page
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Milkey Mouse
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3 changed files with 120 additions and 119 deletions
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@ -12,3 +12,4 @@ This chapter details deployment strategies for the following scenarios.
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deployment/central-backup-server
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deployment/hosting-repositories
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deployment/automated-local
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deployment/image-backup
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119
docs/deployment/image-backup.rst
Normal file
119
docs/deployment/image-backup.rst
Normal file
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@ -0,0 +1,119 @@
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.. include:: ../global.rst.inc
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.. highlight:: none
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Backing up entire disk images
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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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Decreasing the size of image backups
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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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Using ntfsclone (NTFS, i.e. Windows VMs)
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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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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 $x
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done
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Restoration is a similar process::
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borg extract --stdout repo::hostname-partinfo | dd of=$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 "$PARTITION" | cut -d' ' -f1)
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fi
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done
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.. note::
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When backing up a disk image (as opposed to a real block device), mount it as
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a loopback image to use the above snippets::
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DISK=$(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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Using zerofree (ext2, ext3, ext4)
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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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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 $DISK
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Because the partitions were zeroed in place, restoration is only one command::
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borg extract --stdout repo::hostname-disk | dd of=$DISK
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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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- 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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Virtual machines
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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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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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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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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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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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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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119
docs/faq.rst
119
docs/faq.rst
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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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Yes, the `deduplication`_ technique used by
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|project_name| 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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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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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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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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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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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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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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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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How can I decrease the size of disk image backups?
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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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Using ntfsclone (NTFS, i.e. Windows VMs)
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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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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 $x
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done
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Restoration is similar to the above process, but done in reverse::
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borg extract --stdout repo::hostname-partinfo | dd of=$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 "$PARTITION" | cut -d' ' -f1)
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fi
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done
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.. note::
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When backing up a disk image (as opposed to a real block device), mount it as
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a loopback image to use the above snippets::
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DISK=$(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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Using zerofree (ext2, ext3, ext4)
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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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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 $DISK
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Because the partitions were zeroed in place, restoration is only one command::
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borg extract --stdout repo::hostname-disk | dd of=$DISK
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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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- 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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Can I backup from multiple servers into a single repository?
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------------------------------------------------------------
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