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I had about 8.9GB hard disk space before installing Snow Leopard (had Leopard before). After installation, the hard disk space increased to 20GB.

What caused this huge jump? I know some webpages says it is because the way Apple calculates disk space, using 1,000,000,000 instead of 2^9 for a GB, but the difference should be only 7.4%

2^9 = 1,073,741,824

So, why did 8.9GB of free space become 20GB? 8.9 x 1.074 = 9.56GB, so why is there an extra 10.5GB? Even if some printer drivers were deleted, they should only take up tens of MB, not 10GB.

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3 Answers

up vote 15 down vote accepted

John Siracusa’s review of Mac OS X 10.6 lists several space freeing features (on page 3). The biggest contributors are “install on demand components”, dropping support for PowerPC processors, and “HFS+ compression”.

  • The Rosetta PowerPC binary translator software and the bulk of the printer drivers and are not installed by default.

    They can be installed from DVD or downloaded when they are needed.

  • The system-installed programs have dropped support for PowerPC processors.

    At least one architecture was eliminated from every program on the system (32-bit PPC). Some of them dropped two architectures (32- and 64-bit PPC). This means that most Snow Leopard executables only store code for half the number of ISAs as the equivalent Leopard binaries. The different ISAs take different amounts of binary code to represent the same high-level program, so this does not always mean a 50% space savings for each binary. Nevertheless, “50% byte savings for each executable” is a fairly accurate estimate.

  • Many of the Snow Leopard files are also transparently compressed.

    Limited access to HFS+ compression is provided in through the ditto(1) and afscexpand(1) commands. Some third part tools provide ways to identify or find such compressed files, but this is generally not a “user level” feature.

The review also mentions the change to powers-of-ten for reporting large byte sizes (1 GB = 1,000,000,000 (10^9) bytes instead of 1,073,741,824 (2^30) bytes) in the “A gigabyte by any other name” sidebar.

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The main reason Snow Leopard frees up so much disk space is that it discards all PPC code from the system and system applications when it is installed.

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really... it was an Intel macbook to begin with, so all the PPC code was there for a mistake? –  動靜能量 Apr 3 '11 at 15:43
    
No, not a mistake. Mac applications can bundle together multiple versions of executable for a variety of useful reasons. In Snow Leopard, Apple decided to cut out PPC code. –  Wayne Apr 3 '11 at 16:36
    
No, Thanks to Rosetta you can run PPC App on Intel based Macs. –  Rabskatran Apr 6 '11 at 10:45
    
To clarify it doesn't strip any third party or old Apple applications of PPC code it just replaces System Applications and files with Intel-only binaries. Rosetta is an optional download for Snow Leopard to aid in Intel transition. It is recommended to replace PPC applications with Intel native applications as Rosetta will no longer be supported after Snow Leopard. –  iklier Apr 29 '11 at 2:25
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The printer drivers are hundreds of megabytes, if not over a gigabyte. Granted that doesn't account for all of it. I would venture a guess that you had quite a bit of cache, temporary files and other junk that was deleted in the upgrade. For example, a while ago I deleted my Adobe Bridge cache and it was 8 gigabytes.

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no... it is said that Snow Leopard always gives back a lot of hard disk space, even for people who have not much cache or tmp files at all. –  動靜能量 Apr 3 '11 at 3:27
    
I've upgraded 3 Macs to Snow Leopard and none of them had that much space freed up. But even if my answer isn't helpful, the printer drivers account for much more than you credit. –  emgee Apr 3 '11 at 3:38
    
the official answer I got from Apple Store was it free up about 8GB... so how much did you free up? –  動靜能量 Apr 3 '11 at 3:41
    
If I recall correctly about 10GB each for the two Mac Pros and about 6GB from the MacBook Pro, but the MacBook Pro didn't have printer drivers installed on it. –  emgee Apr 3 '11 at 3:44
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