MacOS – Backing up to an existing Time Machine sparsebundle

Time Machine works at a file level, with no facility to perform incremental changes within files. As such, your Sparsebundle may be backed up in it's entirety every time it changes in the slightest, depending on how large it is. Of course, you have to wait up to 1 hour (+the time it take to make the backup, depending on how large the queue is and where your file sits in it) to ensure those changes are included in the backup. Also if your Sparsebundle is in use (mounted...) then it may well skip it until the file lock is freed

This is a terrible system, and one that we might not see change until the underlying filesystem is suitable upgraded (or replaced) to include such useful facilities as incremental block level changes rather than simple file level ones, and/or deduplication etc. One early victim of this scenario were users who used the original Filevault system for encrypting their home folders. Time Machine would not backup their home folders until such time as they logged out because the Sparseimage file was constantly locked by the fact the user had it mounted. And even when the user did log out, it would proceed to make a hugely inefficient backup of the whole thing again and again - on the assumption that they simply logged out and didn't just turn off etc... Not very clever. To try to ameliorate this the Sparseimage spec was ammended to allow for Sparsebundles. Instead of a single big file, a sparse bundle is a bundle (directory) containing a number of files called bands, each in the order of 8 MB in size. This means even though to the end user the sparse bundle appears as a single file, it is composed of smaller files. As of Mac OS X 10.8, the bands are 8.4 MB each. When the content of the image changes, one or more band files is changed, created, or deleted. This allows backup software (such as Time Machine) to operate more efficiently, but it's just a bodge to attempt to mimic block level changes in individual files, which is limited to 8Mb "blocks"...

So to answer your questions directly, 1) it handles them properly, where properly means the same as any other file, it's just that your particular use (leaving it open and mounted) may not result in efficient backups that are taken regularly, especially if you rarely unmount the file, and 2) yes, you will need to pull back the whole file to view it's contents. The TM restore interface is also file specific. It may have quick-look plugins to allow you to view simple files inline like JPGs etc, but not for a complex file like a sparsebundle.

On the bright side, you already have a licensed copy of ChronoSync, which is super useful, and I would continue to use this to perform incremental backups of your sparsebundle whilst it is mounted, you can use the same drive as your TM images too.

These instructions assume you're using a sparsebundle file in 10.9 because you use an encrypted drive. If not, modify accordingly.

First, follow the instructions in the link below to figure out how to use security find-generic-password to grab the password to your encrypted drive out of your Keychain programmatically.

http://blog.joshdick.net/2012/10/14/programmatically_mounting_encrypted_disk_images_in_os_x.html

Then create a file called mounter in your local bin directory ~/bin that contains the following code (replace all paths with your paths, and in the security find-generic-password, use the name of the key in Keychain):

#!/usr/bin/env bash -e

PHYSICAL_DRIVE_PATH="/Volumes/Backup/"
SPARSEBUNDLE_PATH="/Volumes/Backup/TimeMachine.sparsebundle"
SPARSEBUNDLE_MOUNT_PATH="/Volumes/Time Machine/"

# Check existing states
if [ -e "$SPARSEBUNDLE_MOUNT_PATH" ]; then
    echo "Already mounted."
    exit 0    
fi

if [ ! -e "$PHYSICAL_DRIVE_PATH" ]; then
    echo "Physical drive not attached"
    exit 0
fi

if [ ! -e "$SPARSEBUNDLE_PATH" ]; then
    echo "Virtual drive not found on physical drive"
    exit 1
fi

# The mount command uses security find-generic-password
# to get the password from the keychain store
MOUNT_PASSWORD=$(security find-generic-password -w -D "disk image password" -l TimeMachine.sparsebundle)
printf $MOUNT_PASSWORD | hdiutil attach -stdinpass -mountpoint "$SPARSEBUNDLE_MOUNT_PATH" "$SPARSEBUNDLE_PATH"

This is a bit verbose, but means no storing passwords anywhere but keychain. In any case, now we have to get it to run. Create a file in ~/Library/LaunchAgents/ like ~/Library/LaunchAgents/com.martorana.dave.mounter.plist and put the following code in it, replacing, of course, the paths with your own paths:

<?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>com.martoranas.dave.mounter</string>
    <key>Program</key>
    <string>/Users/dave/bin/mounter</string>
    <key>RunAtLoad</key>
    <true/>
    <key>StartInterval</key>
    <integer>60</integer>
    <key>StandardOutPath</key>
    <string>/Users/dave/bin/ldout.log</string>
    <key>StandardErrorPath</key>
    <string>/Users/dave/bin/lderr.log</string>
</dict>
</plist>

We're using launchd instead of cron for a host of reasons, the most important being access to your keychain.

Now load your launch daemon:

launchctl load -w ~/Library/LaunchAgents/com.martoranas.dave.mounter.plist

And that should do it. Now launchd will run your shell script every 60 seconds, and if not mounted and available, will mount your sparsebundle disk image.

Cheers,

Dave