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Once after using Time Machine in Lion Recovery OS to restore a system, I noticed that the attributes B-tree was unusually small.

Initially I was worried (had Time Machine failed to backup some thing, or failed to backup important metadata relating to a number of files?) but before long, I was reassured:

  • the attributes file grew to something like the size it was before erasure.

During the period of growth I was vaguely aware of the system being busy at various times. For a restored system: the business didn't surprise me, but from things like Activity Monitor I gained an impression that not much of the business was Spotlight-related, which sparked my curiosity. When I realised that the attributes file had grown so much, I was very puzzled.

Splitting and rephrasing the question:

What types of data are written to the attributes file?

Answers to this question might give examples of types of data. From the manual page for ls I'm familiar with option -@

Display extended attribute keys and sizes in long (-l) output.

Apple's TN1150 offers technical information, but no examples.

Timing: is some of that data written only after documents and other files have been parsed by the System? 

In a possibly related question, I wonder whether Quick Look preview data (maybe distinct from thumbnail data) is written to an attributes file:

I have fileXray, and am prepared to use it before and after a future Time Machine restoration of a system, but with a good set of answers to this question I might not need to use it.


Technical Note TN1150: HFS Plus Volume Format: Attributes File (2004) is somewhat vague:

… reserved for implementing named forks in the future. … An attributes files has a variable length key and three data record types, which makes it roughly as complex as the catalog file. …


The exact organization of the attributes B-tree has not been fully designed. Specifically:

  • the structure of the keys in the attribute B-tree has not been finalized and is subject to change, and

  • addition attribute's file data record types may be defined. …

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