Matthew Rosewarne on 8 Dec 2007 22:08:39 -0000

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Re: [PLUG] [plug-announce] December 5th, 2007: "What's a file?" presented by Mark Jason Dominus

For the curious, extended attributes (including ACLs) are apparently somewhat 
more complex.  While the kernel has a standard way to access EAs, each 
filesystem implements them quite differently:

The Ext filesystems have various different techniques for storing EAs in one 
data block per inode, and sharing blocks between inodes... or not.

JFS stores EAs in one 64KiB extent per inode, or in the inode itself if small 

XFS and ReiserFS use special data structures to store EAs.

One thing that may or may not be surprising is that ACLs and EAs can impose a 
significant performance penalty when accessing files, since not only does the 
inode have to be read, but also the various EA info located on another part 
of the disk.

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