When I got my Mac, I was not sure what to use and if it was necessary. There are so many different opinions that you really do not know whom to trust. Let us face it, the most important reason to buy such a utility is that you somehow feel the need for it. This is how marketing works, to create need and to bring the solution. Applications with words attached like optimization, automatic detection, improved performance make us feel warm and cozy when we buy and use them.

But we do not want to shell out money just to make us feel better. We want facts and figures and figures is what they give us. Have you noticed that visualization is a key part of every defragmentation software? The better the visualization and stats the software gives the more satisfied you feel. You see nasty red colored blocks, huge gaps and all kinds of color changing and sorting activities. Before your drive looked like a mess and after defragmentation did its job everything feels nicely ordered. It leaves a feeling of relieve like you would have cleaned up your office and now you can be more productive.

With all that said, file fragmentation can cause major performance problems. This a well known fact and that is why you should care about it. File systems like FAT32 and NTFS really need such optimization software for best perfomance. Tools like O&O Defrag v8 for Windows and Diskeeper work and they are worth the money.

Is there something similar for OS X? Well times have changed a lot since Mac OS 9 and even in the OS X releases significant changes have been made. The HFS file system has evolved over the years. Three major improvements to prevent file fragmentation were made. One came with Mac OS X 10.2 where file allocations were delayed. Small allocations were combined into one large allocation in one disk area. The other improvement came with Mac OS X 10.3 which addressed the issue that files grow in size over time. This is a problem because there might not be enough free space at the end of the file to append the data in a continuos stream, that is another reason why file fragmentation occurs. OS X 10.3 can automatically defragment such files while you are working.

OS X 10.3 also introduced a technology called Hot Band. Before we learn about this technology we need to look at the physics of a harddrive. Harddisks rotate with a constant angular velocity for example of 7200 rpm. The data closer to the center of the disk becomes more compressed in terms of physical space and thus the data rate increases. This would be the perfect place for files that need to be accessed very often. Exactly this is what Apple introduced with their Hot Band technology. OS X 10.3+ moves files in and out of this hot zone depending on how frequently the file gets accessed.

Another reason that file fragmentation has become less of a performance issue comes from the read-ahead and write-behind caching. This minimizes data access times which is usually the reason why file fragmentation becomes a problem over time. One thing we also should remind ourselves is that on each system there are lots of small files, for example HTML help files. Even if a disk defragmentation tool shows such files as fragmented there is practically no reason to defragment such files. Such files are rarely accessed and because of their size they can easily be kept in the system cache once they are read.

Altogether OS X 10.3+ has some decent functions to prevent disk performance degradation. This is also the reason why you should never use any OS 9 defragmentation tools even if it is possible because the general file structure of the HFS file system remains the same. But there are some additions that could be damaged if you are using them in conjunction with OS X. So please do not even think of it. But even disk defragmentation tools for OS X are not all equal. For example some applications do not recognize the hot zone that OS X 10.3+ uses, such tools would degrade your file system performance.

To really answer the question “Do you need disk defragmentation tools for OS X?” I even went further in my research. There is only one tool I know of that claims to respect all of the optimizations used in OS X 10.3+. The software is called iDefrag by Coriolis Systems. Their demo version just allows to defrag volumes up to the size of 100 MB. Personally I think this is a bad product demonstration, a 30 day evaluation version would have served better as a demonstration of their software. Instead of 30 days I spent $30 and bought the software to give it a spin.

I have bought my Powermac G5 in November 2005 with a 150 GB harddrive and OS X 10.4. I use my Mac around 10 hours a day, there are over 600,000 files on my disk and there is around 35 GB of free space remaining. The applications I use range from iTunes over the Adobe Creative Suite 2 to Final Cut Studio. If this was a windows machine my usual file fragmentation would range from 6% to 15%. These are just rough estimates, I know of people with much higher file fragmentation on Windows machines. So there should be around 60,000 fragmented files.

What would be your guess for my Powermac with OS X 10.4? Well it was around 1800 files that is 0.3%. Pretty amazing number for a system that has over 10 hours of daily use for over half a year. I make also heavy use of my web browser, at the moment I have 24 open browser tabs. It seems such things do not have much impact on file system performance.

But still iDefrag does such things as disk compaction and metadata optimization. Since most of these optimizations can not be done with your boot drive you need to create a boot cd with iDefrag on it. This process is really painless since Coriolis Systems offers a nice utility that creates a boot cd with the click of a button. I must admit iDefrag has some really nice visualizations and is the only tool I know of that is able to place files on the disk by parameters that you can define yourself. The time defragmentation takes depends on many parameters. I checked most of the options that had the word performance somewhere in the context help. So my optimization probably took longer than usual, roughly around 4 hours. So it is really recommended to do such things while you are watching a movie or can do something that is not related to your Mac.

So was it worth it? Well, yes and no. My files look really well sorted now. My fragmented file count went down to around 120 files. iDefrag does what it claims and it does it well. But were there any noticeable improvements? Not really, boot up time was the same as always and the start up after the user login did not speed up by any noticeable amount. My applications did not start any faster, so there was really nothing that would have convinced me to buy iDefrag if there had been a 30 day demo version.

Is this the fault of iDefrag? No. The only time they even mention the word performance on their product page is in conjunction with optimizing disk images before they are burnt onto a CD or DVD. Which probably will bring some performance gains. The one thing they mention which the HFS+ file system is unable to do, is that it does not defrag large files; which iDefrag does. That is it about the claims they are making. The rest of the page is mostly about how they color coded all the information and how you can access all of the file fragmentation information.

I guess after this it is safe to say that for most people disk defragmentation tools for OS X are not needed. OS X 10.3+ does an amazing job in handling file fragmentation and you can rely on these optimizations. If you are using an older version of OS X you really should consider to buy an upgrade because there a major improvements in the latest version not only within the file system. Ars Technica has written an in depth review of Mac OS X 10.4 Tiger.

One general hint to minimize file fragmentation on every OS is to use at least two drives. Always try to swap data from one drive to the other. For example if you are editing a large video file you should put the source files on one disk and render the output to the second disk. If you want to install a large application from a downloaded archive, make sure that the archive is not on the same disk that you want to install it on. This way you help the system to get a continuos stream of data that can be written in a continuos stream to the second disk. Also this improves disk performance a lot since the harddrive does not need to seek back and forth between source and target files.