MacOSX Leopard - Time Machine

Andrew Stephens, Tuesday the 1st of July, 2008

Talk about bad luck! The week before Apple's latest and greatest operating system is released, my MacBook hard drive decides it can no longer go on living This post was automatically imported from my old sandfly.net.nz blog. It may look a little weird since it was not originally written for this format.. As I result, I have lost a years worth of email and photos just days before. If the hard disk had waited just a few more days then everything would have been safely backed up using the new Time Machine feature in MacOSX.5.

Let me begin by saying that Time Machine is not the last word in backing up, you can definitely buy better software. However, Time Machine may well be the prettiest piece of software ever created for any purpose whatsoever, so much so that I am actually looking forward to accidently deleting something in the future. Time Machine Restore Screen
Restoring files with time machine, pity you can't see the moving starfield in the background

Although it looks great, and works well, Time Machine is a little strange compared to other backup tools. The Mac filesystem has no equivalent to Windows' Volume Shadow Services, so you cannot literally roll back a folder transparently. What Time Machine does is take periodic snapshots of your entire hard disk and stores it on another volume, usually some type of removable USB drive.

The first backup you take is basically a copy of your entire filesystem (the files are not compressed and can actually be accessed in the finder). Every half hour after that, time machine will create a new backup directory and copy any files that have changed.

Much of the implementation is done with UNIX-style links to directories, so that files that have not changed since the last backup are not re-copied. This has some important implications (both good and bad) that I have not seen mentioned anywhere else:

GOOD: The entire filesystem is stored so it is simple to do a complete recovery if your entire hard disk fails.

GOOD: The backup directory structure is so simple that you don't even really need the fancy flying-through-space GUI to get your files back.

BAD: Links to folders are not supported on any other filesystem other than MacOS. This means you have to reformat your backup drive. Which leads to:

BAD: Very limited network support. Wouldn't it be awesome to have everything backed up to a network share? Why yes, yes it would! Well you can't - unless your network share supports links to folders - which it doesn't (well, if you are running your share off Leopard Server it does - but you get the point).

BAD: Backups are only done every half an hour hourly, and only 1 backup is kept for each day earlier than 24 hours ago. Other backup tools backup every file as soon as it is saved.

GOOD: Unless you are stupid and backup to another partition, your backups are on a different physical drive that will hopefully survive whatever calamity that destroyed your data.

Although it is not the mythical perfect backup software, Time Machine is a very useful addition to MacOS. The very fact that it is bundled with the OS and very easy to set up is sure to save many hours of tears and frustration for Mac users.