What Time Machine Does

Time Machine is a very basic program at heart. In essence, it copies files from your system to the backup destination of your choosing. Once the program is properly configured and the initial backup is complete, updates occur once every hour that the system is on. As long as the chosen destination is available, updates occur in the background without user intervention. On modern systems you probably won’t even notice that a backup is taking place. It’s a bit like having a personal assistant writing down everything that has happened to your system in the last hour.

How It Works

If that’s all there were to Time Machine, I probably wouldn’t find it very interesting. After all, at this level it’s essentially performing hourly, incremental backups throughout the day. The ingenious trick is using the hourly backups as the basis for a file archive. At the end of a day, hourly backups are combined into a daily backup. Let’s suppose I accidentally delete an important paragraph from this article just before the backup is executed. Later, when I realize the mistake, I can revert to a version…which still contains that brilliant missive.

Time Machine’s hourly backups are kept for 24 hours before it starts deleting older versions to save space. The first backup of the day is generally kept for an entire month. The first backup of the week is kept until your chosen backup location is almost full. In a further attempt to save space, Time Machine makes use of “hard links.” Rather than repeatedly copying files that haven’t been modified, Time Machine uses a “hard link” to point back to the original copy of a file (or folder). A new copy of a file is only written when that file is modified.

What Time Machine Does Not Do

Perhaps as a concession to simplicity and ease of use, Time Machine has a few limitations. There are some things it was never designed to do, and these limitations must be considered if it is to be used effectively. Some of these limitations can be addressed with a little thoughtful preparation, but other issues must be addressed with additional software. The most important of these limitations include:

Setting Up Your First Time Machine Backup

One of the first decisions you will have to make when setting up a Time Machine backup is where to place the files. Time Machine will back up to almost anything except optical drives, iPods or iDisks, and hard drives connected to AirPort Extreme Base Stations. External hard drives, Time Capsules, even network shares are all fine as far as Time Machine is concerned. Whatever your chosen destination, make sure it has plenty of space and is formatted as HFS+ with journaling enabled. You can also watch a visual guide to the setup process.

Although Time Machine can share space on the same drive with other files if you like, it won’t let you place a backup on the same drive partition that your Mac boots from. If your boot drive has a second partition, Time Machine can make use of that space, but I don’t recommend it. Any failure of the drive could not only compromise your ability to boot the Mac, but also your ability to restore files from your backup.

Now that you’ve got that hard drive connected, launch Time Machine via the Time Machine application or via the Time Machine preference pane. In a moment you are presented with the main Time Machine screen.

Click the “Choose Backup Disk” option. This brings up a sheet that allows selection of the desired backup drive. All storage devices attached to your Mac are listed here. If a drive is not properly formatted for Time Machine’s use, the message “Reformat Required” appears beside that device. Choose a location, then choose “Use for Backup.”

Once a useable destination is selected, Time Machine takes you back to the original configuration screen. Notice that the right side of the screen now shows the name of the device, the available space, the date of the oldest backup, and the date of the latest backup. The time before the next backup is also shown here. There’s also an option to put a Time Machine status indicator in the menu bar. With this indicator, you have an easy means of forcing an immediate backup, launching Time Machine, or opening Time Machine’s preferences.

Putting Time Machine on a Diet

Now that we are back to the main screen, our configuration of Time Machine is almost complete. Remember when I said that by default Time Machine backs up the entire source drive? If that’s what you want to do, your configuration is essentially done. If, on the other hand, you’d like a little control over the size of your Time Machine backup, now is the time to address that problem. Controlling the size of a backup is a little bit like trying not to gain weight. It’s much easier to keep the extra weight off in the first place than to take it off later. Although you cannot exclude files from a backup based upon rules, it is possible to exclude files and folders.

Now that you’ve designated a backup disk, you should be back to the Time Machine preference pane. Click the “Options” button to bring up a file selection sheet. Click the “+” sign to select a file or folder to be excluded. Continue doing this until all unneeded files are excluded.

Since I tend to keep a reasonably current system clone around to boot from in an emergency, I didn’t worry about having Time Machine back up system files. I also excluded the Applications and Developer Tools. Keeping a current clone means I didn’t really need another copy of the Applications folder and the Developer folder, which includes roughly 3 GB of files that are not updated often. Following these steps, my current Time Machine backup currently extends two months into the past and takes up just under 84 GB of drive space.

Other types of files can also cause the size of a Time Machine backup to explode. The Entourage e-mail database and disk images used by virtualization software are good examples. Not only are these often large files, in some cases several gigabytes of data, but changes are made almost every time these programs are used, necessitating a backup of the file rather than creation of another hard link.

Overcoming Limitations—The Bootable Backup

I must admit to being a little disappointed to discover that Time Machine backups were not bootable. This creates a situation that limits Time Machine’s usefulness. As long as your boot drive is OK, restoring from a backup is easy: launch Time Machine and locate the file that you want to recover, select the file, then click the “Restore” button.

If your boot drive has had a meltdown, though, this slows down the recovery process. In that case, Apple seems to expect you to boot from the OS X install DVD to perform the restoration, which can be quite time consuming. The solution is to make sure your designated backup drive is bootable.

If you are starting with a fresh drive, try installing and updating a copy of the OS before designating that drive as your Time Machine volume. In theory, other utilities could be added as well to create a “recovery drive” that has everything necessary either to get the existing boot drive hale and hearty or to move the data to a new drive via Time Machine. I’ve taken that one step further by keeping a current system clone on one partition of the drive that has my backup. In an emergency, I can boot from that drive and have ready access to the software I need as well as current data.

If you are using a drive that already has data on it, you can still create a bootable backup. This process involves using a cloning tool such as Carbon Copy Cloner to clone your install DVD onto the drive that contains your Time Machine backup. Set it to copy everything, and not erase the target volume. This provides all the benefits of booting from a DVD with the speed of a hard drive.

Overcoming Limitations—Creating Encrypted Backups

As far as I can determine, there isn’t an efficient way to create an encrypted backup of your data. About the best you can do at the moment is to encrypt files or folders and let Time Machine back up the resulting data. You cannot restore these files from directly within Time Machine. In a February 2008 article, Macworld concluded “…you can manually mount a Time Machine disk image and use the Finder to copy a single file if you wish.”

Overcoming Limitations—A Matter of Timing

By default, Time Machine backups occur hourly as long as the computer is on and the target device is available. That time frame suits my needs well, but what if you want to follow a different schedule? Apple doesn’t provide an option for setting the backup interval through the normal Time Machine interface. By now, you must know me well enough to know that I wouldn’t have broached the subject if I didn’t have a possible answer. If you like to use Terminal for such tasks, fire it up and enter the following command. Since you are using sudo, be prepared to enter an administrator password.

sudo defaults write /System/Library/LaunchDaemons/com.apple.backupd-auto StartInterval -int 1800

The number at the end of this command represents the time interval, in seconds, between backups. Executing the command as written above would change the interval between backups to 1800 seconds, or 30 minutes. Thanks to Cory Bohon at The Unofficial Apple Weblog for that tip. If you’re a bit leery of Terminal check out Time Machine Scheduler as one possible alternative. I haven’t tested it yet.

Overcoming Slow Backups on Wireless Networks

Let’s suppose you are backing up your favorite laptop via Time Machine and something like Apple’s Time Capsule. Some users have reported that the backup is incredibly slow. This is particularly true for the initial backup, when a large amount of data is being copied. For the initial backup, seriously consider using a wired connection. If updates over a wireless network are still slow, it may be due to a mixed wireless 802.11b/g/n network.

Restoring Files From a Backup

Restoration from a Time Machine backup is about as simple as these things get. Launch the Time Machine application. When the application opens, it consumes your entire screen with a time line on the right side of the screen. Restore and Cancel buttons dominate the bottom of the screen with a window representing the state of the drive in the center of the screen.

Restoring a file is simple. Click the time line on the right to get to the date you are interested in examining. Windows representing your drive will fly by until the correct one is reached. This is a cool effect that’s almost worth having to restore a file. Select the file you wish to restore in the window and click the Restore button. If you are not sure you have chosen the correct file, select one and tap the spacebar. Quick Look comes to the rescue, making it possible to preview the file before completing the restoration. Should you need to cancel a restoration, either choose Cancel or press Escape.

Final Thoughts

For a few months, I had an occasional error where Time Machine would inform me that a backup could not be completed. When the error occurred, the next backup usually completed without incident, regardless of whether it was initiated automatically or manually. I haven’t experienced this error recently, but I’m not sure if that is a function of recent system updates or my switch from the Mac Pro to the MacBook Pro as my main machine.

Time Machine has worked nicely for me during the last several months, partly because I’ve integrated it into a larger strategy that includes cloning my primary drive to get back up and running a bit quicker than if I needed to boot from the Leopard DVD for a restoration. For most users, Time Machine can be an effective component in a backup regimen that includes consideration for off-site backup and booting quickly in an emergency.

If you don’t have a current backup strategy, Time Machine may meet your needs. It can’t back up to optical media, and restoration requires either a little advance planning or a convenient bootable OS DVD. Available disk space may also be a concern, but Time Machine almost makes backup and restoration fun.

Copyright © 2009 Sylvester Roque, sroque@atpm.com.