OSX tar: restored files larger on disk

[greno]

Can someone please explain why OSX bsdtar and gnutar are creating restored files larger on disk than when they were archived?

Here's an example:

Mes-Mac-mini:test root# gnutar -czf /tmp/XPC-MacOS.tgz -C / System/Library/XPCServices//com.apple.cmio.registerassistantservice.xpc/Contents/MacOS 
Mes-Mac-mini:test root# gnutar -xzf /tmp/XPC-MacOS.tgz 
Mes-Mac-mini:test root# 
Mes-Mac-mini:test root# du -sh System/Library/XPCServices//com.apple.cmio.registerassistantservice.xpc/Contents/MacOS
 52K    System/Library/XPCServices//com.apple.cmio.registerassistantservice.xpc/Contents/MacOS
Mes-Mac-mini:test root# 
Mes-Mac-mini:test root# du -sh /System/Library/XPCServices//com.apple.cmio.registerassistantservice.xpc/Contents/MacOS
 12K	/System/Library/XPCServices//com.apple.cmio.registerassistantservice.xpc/Contents/MacOS
Mes-Mac-mini:test root# 
Mes-Mac-mini:test root# 
Mes-Mac-mini:test root# ls -la System/Library/XPCServices//com.apple.cmio.registerassistantservice.xpc/Contents/MacOS
total 104
drwxr-xr-x  3 root  wheel    102 Jul 31  2011 .
drwxr-xr-x  3 root  wheel    102 May  6 16:17 ..
-rwxr-xr-x  1 root  wheel  50464 Feb 20 15:42 com.apple.cmio.registerassistantservice
Mes-Mac-mini:test root# ls -la /System/Library/XPCServices//com.apple.cmio.registerassistantservice.xpc/Contents/MacOS
total 24
drwxr-xr-x  3 root  wheel    102 Jul 31  2011 .
drwxr-xr-x  7 root  wheel    238 Jul 31  2011 ..
-rwxr-xr-x  1 root  wheel  50464 Feb 20 15:42 com.apple.cmio.registerassistantservice
Mes-Mac-mini:test root#

As you can see the restored files from the OSX gnutar archive take up nearly 4.5 times as much room on disk, 52K v 12K.

OSX bsdtar suffers from the same problem.

What is causing this?

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[greno]

The answer is that the files under /System are being compressed in newer versions of HFS+. And when you archive and extract them again then they become uncompressed and therefore larger on disk.

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