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Forcing Time Machine to Use a Local Volume as a Network Volume

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Time machine uses a feature of the HFS+ filesystem that was introduced
in Mac OS X 10.5 (Leopard) called _directory hard links_. Like file hard
links, a directory that is hard linked to another directory is not
actually a distinct directory, but is instead a pointer to the original
directory. Time Machine uses these directory hard links to reduce
duplication and save space by making references to entire directory
trees whose contained files have not been modified. 

To properly copy or duplicate a Time Machine backup, these directory
hard links must be preserved. Unfortunately, directory hard links are
proprietary to Apple. Apple discourages their casual use by third party
developers because, if used incorrectly, they could create recursive
directory structures that would render a volume effectively useless.
This introduces obstacles to anyone wishing to make copies of a Time
Machine volume.

Time Machine may be configured to back up to a local volume that is
connected directly to a Mac (either internally, or externally with a
Firewire or USB connection), where it creates a Backups.backupdb folder
at the root of the volume, storing all of the backup data in the
volume’s native file system using the previously mentioned directory
hard links. Unfortunately, Time Machine will not back up to a local
directory, but instead requires an entire volume be dedicated to
backups. Further, Time Machine will utilize as much free space as
possible on the volume. To prevent Time Machine from filling a disk, you
could partition the disk to create multiple volumes, dedicating only one
of the partitions to Time Machine backups. Yet you would still have no
easy way to copy the backup data off the partition, due to the directory
hard links.

Time Machine may also be configured to back up to volume that exists on
a remote Mac and is mounted locally as a shared network volume. In this
configuration, Time Machine creates a sparse disk image bundle which
contains its own internal file system where backup data is stored. Since
the backup data (with directory hard links) is stored within a
self-contained disk image, one can easily make a copy of the entire disk
image without worrying about effecting the directory hard links
contained within the disk image. For this reason, backing up to a shared
volume could be preferable for those who don’t want to dedicate an entire
volume to backups, or who want to be able to easily make file system
level copies of the backup data for added security.

So what if you want to prevent Time Machine from using all available
free space on a _local_ volume and also back up to a _disk image_? This
procedure shows you how to do that. You can make an AppleShare network
connection to the same machine by adding an extra IP address to the
local loopback interface as an alias. This allows you to treat any local
directory or volume as if it were a remote shared network volume, so
that Time Machine will create a sparse disk image bundle on it which can
easily be copied.

1. Use the System Preferences > Sharing > File Sharing facility to
create a shared folder on any local volume. This folder will hold your
Time Machine backups.

2. From an administrative account, open a terminal window and enter
this command to create a loopback alias to the 127.0.0.2 IP address: 

# sudo ifconfig lo0 alias 127.0.0.2/32

2. Verify the loopback alias was created by looking for the 127.0.0.2
address in lo0 interface section of the ifconfig output:

# ifconfig lo0

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
	options=3<RXCSUM,TXCSUM>
	inet6 ::1 prefixlen 128 
	inet 127.0.0.1 netmask 0xff000000 
	inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1 
	inet 127.0.0.2 netmask 0xffffffff             <--
	nd6 options=1<PERFORMNUD>

3. Use the Finder Go > Connect To Server menu command to connect to the
server: afp://127.0.0.2, and select and mount the shared folder you
created. You should see the shared folder mounted as a shared volume
icon on the desktop.

4. Use the System Preferences > Time Machine facility to add the mounted
shared volume as a Time Machine backup destination. Once this is done
you can eject the shared volume from the desktop. Time Machine will
mount and eject the volume automatically when needed in the future.

5. To create the loopback alias address each time the machine is booted,
create a Launch Daemon property list file at
/Library/LaunchDaemons/my.domain.loopback.plist with this content:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
	<dict>
		<key>Label</key>
		<string>my.domain.loopback</string>
		
		<key>Disabled</key>
		<false/>
		
		<key>RunAtLoad</key>
		<true/>
		
		<key>ProgramArguments</key>
		<array>
			<string>/sbin/ifconfig</string>
			<string>lo0</string>
			<string>alias</string>
			<string>127.0.0.2/32</string>
		</array>
	</dict>
</plist>

Now Time Machine will happily back up to a sparse disk image bundle on
the local shared folder or volume.

I’ve been using this solution for a few months now, including
successfully restoring several items from my backups, without any
apparent issues.

Disclaimer: This information is offered with no guarantee of support
whatsoever (unless freely and unexpectedly given, purely out of the
grace and excellent basic goodness-of-heart of said author, who makes
absolutely no claims whatsoever to promise same). Nevertheless, and with
all serious intentions of being a "fine person of some reasonable degree
of earnestness, kind-hearted interest, human understanding and
worthwhile character," said author would like to hear from you
concerning your excellent or other adventures or misadventures with this
information.

-- 
E-mail sent to this address may be devoured by my ravenous SPAM filter.
I often ignore posts from Google. Use a real news client instead.

JR

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Thread

Forcing Time Machine to Use a Local Volume as a Network Volume Jolly Roger <jollyroger@pobox.com> - 2014-03-07 15:16 +0000
  Re: Forcing Time Machine to Use a Local Volume as a Network Volume Bread <BreadWithSpam@Fractious.net> - 2014-03-11 09:11 -0700
    Re: Forcing Time Machine to Use a Local Volume as a Network Volume Ted Lee <tmplee@gmail.com> - 2014-03-11 11:42 -0500
      Re: Forcing Time Machine to Use a Local Volume as a Network Volume David Sankey <David.Sankey@stfc.ac.uk> - 2014-03-12 13:35 +0000

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