I wanted to determine how much space I’d save—and what the compression time tradeoff might be—to compress Mac OS X system images with gzip versus bzip2. Both are options in Mac OS X Tiger, but the bzip2 compression is new in that release (and thus not backward compatible with Panther and earlier). Given that Apple’s own disk imaging is the bedrock of almost all imaging solutions from Mac OS X, including those from third party computer management solutions providers, this information is valuable and widely applicable on the Mac platform.
If the time were the same but I saved some disk space, I’d count that as a win for one compression choice or the other. For significant space savings, I’d even sacrifice time to a longer conversion operation because disk image compression and ASR prep can be handled in the background during downtime or operational lulls. A Folder Action or launchd task might be ideal for that, and might even be a useful option if you are just archiving images.
The two disk image formats I tested are UDZO and UDBZ, as defined by the hdiutil command and its man page.
For an UDZO version of the image (gzip compression at level 9—or best),
I got the following results from hdiutil convert:
Elapsed Time: 1h 1m 50.751s
File size: 4226869141 bytes, Checksum: CRC32 $8CF3F127
Sectors processed: 20184610, 19980489 compressed
Speed: 2.6Mbytes/sec
Savings: 59.1%
I ran the hdiutil command with time (a handy utility if you haven’t used it), to get these figures:
real 61m52.133s
user 51m7.575s
sys 2m47.700s
For the same image converted to UDBZ (bzip2 compression):
Elapsed Time: 2h 16m 28.057s
File size: 3956667545 bytes, Checksum: CRC32 $8CF3F127
Sectors processed: 20184610, 19980489 compressed
Speed: 1.2Mbytes/sec
Savings: 61.7%
The time results for that were:
real 136m29.205s
user 106m1.832s
sys 8m30.739s
You may draw your own conclusions, as appropriate to the data and your situation. The source image was the same size, nearly 10 GB, which included Mac OS X Tiger and a suite of applications with a small number of user accounts. The source and converted files were both stored on an external FireWire 400 hard disk, and converted with a 1.5-GHz 12-inch PowerBook G4.
Although the ASR-prepped (i.e. asr --imagescan) gzip and bzip2 images were not that different in the end, the bzip2 version definitely came out smaller. It has in ever test I’ve ever run. However, it did take twice as long to create the image. So, if you’re strapped for disk space to collect your images and CPU time means less to you than storage (which is often the case for me, given that I have access to a lot of G4-class Mac hardware), I would consider bzip2. However, gzip at level 9 is perfectly acceptable (still providing almost 60% savings in this case), is faster to create, and has the benefit of working on older versions of Mac OS X, as well.
I should try gzip at other settings (like levels 1 and 6, for example), because you can’t choose level 9 in the Disk Utility interface; you must use the command line -imagekey flag. (You can get bzip2 from the GUI if you enable the hidden advanced disk image formats, but I still don’t see a way to choose a higher gzip level.)