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Some applications can store Quick Look information (previews and thumbnails) within the file. Others can not.

Two aspects to this question

For a file that is local and without integral Quick Look information:

  • where does Lion store that information?

For a file that is remote, on an AFP server (Mac OS X Server (Leopard)/AFP 3.2+ or greater) and without integral Quick Look information:

  • does Server store the information remotely?

(An AFP client can cache some information locally, with limitations, but I strongly suspect that there must be a more permanent store.)

Initial guesses

Much Quick Look information is stored in the attributes B-tree.

If so, an attributes B-tree in a more modern version of Mac OS X Server may be potentially better (than older versions of Server) for AFP clients where bandwidth is limited.

Please, does anyone have more definite answers?

Reference and other items of interest

Quick Look — Wikipedia

Mac OS X v10.6 to v10.7 API Diffs: QuickLook Changes

Apple Filing Protocol Programming Guide: AFP Client Caching: File Metadata Caching

  • I don't know whether Quick Look-related information is classed as metadata for AFP client caching purposes.

Technical Note TN1150: HFS Plus Volume Format: Attributes File, in particular:

IMPORTANT: 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.

Quick Look Programming Guide: Quick Look Architecture

Quick Look Programming Guide: Saving Previews and Thumbnails in the Document

In Ask Different:

There are various other Quick Look-related questions in Ask Different and Super User but at a glance, none of them answer this question. Examples:

No significant amount of Quick Look-related information in application support or cache areas

[macbookpro08-centrim:~] gjp22% date
Fri  5 Aug 2011 08:33:02 BST
[macbookpro08-centrim:~] gjp22% uname -a
Darwin macbookpro08-centrim.home 11.0.0 Darwin Kernel Version 11.0.0: Sat Jun 18 12:56:35 PDT 2011; root:xnu-1699.22.73~1/RELEASE_X86_64 x86_64
[macbookpro08-centrim:~] gjp22% du -sh ~/Library/Application\ Support/com.apple.QuickLook/
4.0K    /Users/gjp22/Library/Application Support/com.apple.QuickLook/
[macbookpro08-centrim:~] gjp22% du -sh ~/Library/Caches/com.apple.quicklook.ui.helper/
 52K    /Users/gjp22/Library/Caches/com.apple.quicklook.ui.helper/
[macbookpro08-centrim:~] gjp22% du -sh ~/Library/Caches/com.apple.QuickLookDaemon
 52K    /Users/gjp22/Library/Caches/com.apple.QuickLookDaemon
[macbookpro08-centrim:~] gjp22% du -sh ~/Library/Caches/com.apple.QuickLookDaemon32/
 52K    /Users/gjp22/Library/Caches/com.apple.QuickLookDaemon32/

Meta

Quick Look (two words, in Finder and elsewhere) seems more proper than QuickLook.

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2 Answers 2

Graham - quick look generate the data on the fly. The inspectors know to use the previews when present, but will do the heavy lifting when needed.

The virtual memory system of course caches these files so it feels faster after the first render until it swaps out or gets disposed if the pages are not marked for a swap to disk.

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Thanks. I'll leave this question open for a while (not indefinitely) for three reasons. First: use of virtual memory only for caching might be true if FileVault 1 alone is used. Second: wondering whether there'll be more to learn after iCloud is released, probably with a corresponding update to Mac OS X. Third: Why is the HFS Plus attributes file unusually small, for only a while, after using Time Machine to restore a Lion system? –  Graham Perrin Aug 12 '11 at 9:42
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com.apple.QuickLook.thumbnailcache

In combination with qlmanage, using fs_usage and fseventer drew attention to the following directory:

/private/var/folders/*/*/C/com.apple.QuickLook.thumbnailcache/

Whether this is used for previews as well as thumbnails, I don't know.

A Google search for com.apple.QuickLook.thumbnailcache found (amongst other things) FileVault and QuickLook leak some information from encrypted volumes (2010-07-25) with a comment,

Thumbnail cache is disabled for encrypted volumes, including file vault users. You should be able to confirm with following terminal command:

qlmanage -m disks

For FileVault 2 in Lion, that may be no longer true.

I do have thumbnails.data outside my home directory, 42M, a reset using qlmanage does not reduce its size:

[macbookpro08-centrim:~] gjp22% date
Fri 12 Aug 2011 07:46:24 BST
[macbookpro08-centrim:~] gjp22% uname -mrv
11.0.0 Darwin Kernel Version 11.0.0: Sat Jun 18 12:56:35 PDT 2011; root:xnu-1699.22.73~1/RELEASE_X86_64 x86_64
[macbookpro08-centrim:~] gjp22% ls -h@l /private/var/folders/*/*/C/com.apple.QuickLook.thumbnailcache/
total 87256
-rw-------  1 gjp22  staff     0B 16 Jul 19:42 exclusive
-rw-r--r--  1 gjp22  staff   1.0M 12 Aug 06:35 index.sqlite
-rw-r--r--@ 1 gjp22  staff    18B 19 Jul 00:25 resetreason
    com.apple.TextEncoding    15B 
-rw-------  1 gjp22  staff    42M 12 Aug 07:41 thumbnails.data
-rw-r--r--  1 gjp22  staff   4.0K 12 Aug 06:35 thumbnails.fraghandler
[macbookpro08-centrim:~] gjp22% find /var/folders -name "*QuickLook*" -exec du -h {} \; 2 > /dev/null
find: 2: unknown option
[macbookpro08-centrim:~] gjp22% sh
sh-3.2$ find /var/folders -name "*QuickLook*" -exec du -h {} \; 2>/dev/null
 43M    /var/folders/!!/!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!/C/com.apple.QuickLook.thumbnailcache
sh-3.2$ qlmanage -m disks
disks: 
    /: local fs
    /Volumes/MobileBackups: distant fs
    /home: distant fs
    /net: distant fs

sh-3.2$ date
Fri 12 Aug 2011 07:47:09 BST
sh-3.2$ qlmanage -r
qlmanage: resetting quicklookd
sh-3.2$ ls -h@l ~/Library/Caches/qlmanage
total 104
-rw-r--r--  1 gjp22  staff    52K 12 Aug 07:47 Cache.db
sh-3.2$ ls -h@l ~/Library/Caches/quicklookconfig
total 104
-rw-r--r--  1 gjp22  staff    52K 11 Aug 10:34 Cache.db
sh-3.2$ ls -h@l /private/var/folders/*/*/C/com.apple.QuickLook.thumbnailcache
total 87256
-rw-------  1 gjp22  staff     0B 16 Jul 19:42 exclusive
-rw-r--r--  1 gjp22  staff   1.0M 12 Aug 06:35 index.sqlite
-rw-r--r--@ 1 gjp22  staff    18B 19 Jul 00:25 resetreason
    com.apple.TextEncoding    15B 
-rw-------  1 gjp22  staff    42M 12 Aug 07:47 thumbnails.data
-rw-r--r--  1 gjp22  staff   4.0K 12 Aug 06:35 thumbnails.fraghandler
sh-3.2$ date
Fri 12 Aug 2011 07:48:00 BST

(I use ! above to obscure a path to a private area.)

Also found, amongst help for Cocktail:

  • Manage user caches — useful, but maybe not up-to-date (the list does not include some of the areas mentioned in the opening question).
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