Aaron Adams’s Blowhard Blog

I had some video I captured with iMovie sitting on an external Firewire drive for a little while. That drive was getting full and I needed to clear off some space by moving some of those iMovie projects to DVD. The problem was, one of those projects was 5.38GB, larger than what a DVD can hold. I needed some way to either compress it down or break it up so that it could fit on a single DVD or span multiple discs.

First, I compressed the project using gzip and bzip2 from the command line. Neither utility compressed the video very much, and neither compressed it to a size that would burn onto a single DVD. Enter hdiutil! (The command line analog to Disk Utility.)

As you may know, hdiutil has the ability to compress and segment disk images. I decided to do both of those things and share it here with you, with a couple of additions.

Compressing 5.38GB of data and copying it to a disk image took me some time - 198 minutes. I know that because I used a neat little utility called, appropriately enough, time, whose job it is to print the real time it took a process to complete, the user CPU time, and the system CPU time, at process completion. In this case, time didn’t have any substantive effect on what I did except to satisfy my curiosity. I figured I’d mention it here for your edification.

When a process takes 198 minutes, you may not be at the computer physically or even logged in for that long. You’ll need a way to put that process in the background just in case. That’s where screen comes in. It detaches running processes from a physical terminal (AKA the Terminal application) and lets the user logout and return later while the work you’ve started continues running.

As for the disk image itself, I made a compressed image from a folder. Take a look at this command:

screen -a time hdiutil create -srcfolder /Volumes/Video/Dogs/ -fs HFS+ -volname "The Dogs" -format UDBZ -verbose Desktop/temp.dmg

Wow, that’s a bunch. Here’s the breakdown:

Why did I name the output image temp.dmg? Because after this image is created, we need to segment it, which means writing two new segmented image files while leaving the original intact.

I said screen provided the ability to leave the process running while the user is logged out. Should you need to leave during a multi-hour compression and creation, press control-a and then d afterwards to disconnect. You’ll be returned to the original prompt you started from while hdiutil keeps running. To resume, type screen -r at any time from any command prompt. Should hdiutil finish compressing and creating the image while you’re away, screen will terminate on its own and there will be no session to resume.

After I grew a long white beard, the image was ready. Now it needed to be segmented.

hdiutil segment -o Desktop/Dogs.dmg -segmentSize 4.37g Desktop/temp.dmg

In this case, I ended up with files named Dogs.dmg and Dogs.002.dmgpart. Double-clicking Dogs.dmg mounted the image on the desktop. To mount them in the future, should I need them, I’ll copy both parts to a hard drive and mount it from there rather than from the DVDs.