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Add instructions for ntfsclone (fixes #81)

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Milkey Mouse 2017-11-05 20:42:44 -08:00
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docs/faq.rst
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@ -48,6 +48,64 @@ 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.
How can I decrease the size of disk image backups?
--------------------------------------------------
Full disk images are as large as the full disk when uncompressed and might not get much
smaller post-deduplication after heavy use. This is because virtually all file systems
don't actually delete the data on disk (that is the place of so-called "secure delete")
but instead delete the filesystem entries referring to the data. This leaves the random
data on disk until the FS eventually claims it for another file. Therefore, if a hard
drive 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 of the
VM (or physical computer, if for some reason a normal filesystem backup can't be taken),
there are several ways to decrease the size of a full 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 -
done
# 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 $x
done
Restoration is similar to the above process, but done in reverse::
borg extract --stdout repo::hostname-partinfo | dd of=$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) -
else
borg extract --stdout repo::$x | dd of=$(echo "$PARTITION" | cut -d' ' -f1)
fi
done
.. note::
When backing up a disk image (as opposed to a real block device), mount it as
a loopback image to use the above snippets::
DISK=$(losetup -Pf --show /path/to/disk/image)
# do backup as shown above
sync $DISK
losetup -d $DISK
Can I backup from multiple servers into a single repository?
------------------------------------------------------------