Where Does Finder Store Its Thumbnails?

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I do a lot of work with image files, and Finder does a very nice and very speedy job of thumbnailing them for me. What worries me is that I cannot find out where Finder keeps those thumbnails. Clearly it does keep them, since they seem to be persistent from session to session. Somewhere on my Mac is a repository of these thumbnails that is just getting bigger and bigger and bigger. I would like to find out where it is so that I can flush it from time to time. I have looked in what I consider to be the obvious place (the Caches subdir of my personal Library folder) but I haven't found anything there that would appear to be the set of thumbnails I am looking for.

Has anyone else had this issue? Did you ever find out where Finder keeps these things? Thanks.
 
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im pretty sure it generates the preview 'on the fly' from the file you are trying to preview and then dumps the temp file in a few minutes and not slowly filling up your computer with small thumbnails.
 
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Thanks coach_z. I don't wish to be contrary, but I am pretty sure that this is not the case. I have seen this occur, absolutely. When you go into a new folder of photos that you have not viewed before, you can observe Finder doing just as you suggest. BUT, on the next time you visit that folder, the thumbnails are there instantaneously - WAY too fast to be generated on the fly, which even on my machine you can observe occuring.

I have been using digital cameras forever it seems, and organize my own photo album, on a year by year basis. In any one year's folder, there will be several hundred photos. Finder instantaneously shows me the previews, and I can scroll around, page up and down, and there is no delay at all - they are just there.

Now, if I copy a new set of photos from my camera into a folder and open that, I can observe the thumbnail generation process.

I conclude that somehow, somewhere, Finder is stashing these things. I have read numerous posts with the same question, but no one seems to know the answer.

I hate the idea of a run-away thumbnail cache somewhere, and really want to track this down. Stuff like that can kill a machine eventually.

Any and all pointers much appreciated.
 
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the program disk inventoryx will show you how much space is being taken up by what on your hard drive.
-chris
 
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I can't find anything showing that Finder caches thumbnails on the hard disk. I thought that (like you said) there would be a "Finder" folder in the ~/Library/Cache, but apparently there's not...maybe it caches it on a system level?

I'm beginning to think Finder just stores the thumbnails in RAM...
 
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I may have to take this one to the Expert Desk at the local Apple Store and ask them! Unlike many people, I tend to turn off my Mac fairly frequently and yet these thumbnails seem to persist across power on/off cycles. So, they are stored SOMEWHERE. Just gotta find 'em!
 
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Ah! Got it! I think...

/Library/Caches/com.apple.LaunchServices

I'm fairly certain that this is where Finder's cached items are stored...

Actually, have you tried simply deleting Finder's .plist?
 
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Hmmm... The /Library/Caches thing looks about right - I see that one of these items is owned by my userid and is over a 1 MB already (only had this Mac for three weeks!). However, I tested it out by opening a new folder of over 100 photos that I have never looked at on this Mac. I could see it generating the thumbnails. I checked the size of the /Library/Caches/com.apple.LaunchServices items before and after. Unfortunately, no change, and so that can't be where it is storing them.

Finder's plist looks way too small (less than 4K) for this sort of usage. Anyway, ditto on the size change test after viewing new pictures. As well, isn't a plist a preferences list? Wouldn't I lose all my window settings if I deleted this?

Back to the drawing board?
 
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If you delete the plist then it will lose all finder's settings and preferences and go back to factory defaults.

mac57, since you don't like caches building up save you hunting for the right file why don't you just delete the whole of /Library/Caches and see if it has to recache its images. If not then you have to look in another directory. Once you find it, you can set up a cron job or something to periodically delete them (although i really would have thought OSX does this)
 
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After some more reading around, it's possible that Finder attaches the thumbnail to the image file itself rather than in a cache. Deleting Finder's Plist will clear all of these thumbnails by default.
 
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Hey surfwax95, this sounds promising. Can you point me at a URL where you found this information? It might also help in further information discovery. Ultimately your success with Google is all about asking it the right question!
 
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I honestly don't remember, it was in Apple's docs somewhere. I do remember my search field, however:

"finder cache" +thumbnail +where -10.2

I just had that in my Google bar...
 
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OK, thanks - I'll trudge through some listings.....
 
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No problem. I'm very very certain now that the OS just adds the thumbnail to the resource fork of the image. Still no proof though, I guess...
 
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Interesting! Help out a Mac noobie here. What is a "resource fork"? Is this meta data associated with each file? You are saying that it writes the thumbnail right into the file's metadata, and can use that forever after?

If that is the case, that would be WONDERFUL, and so perfectly Mac'ish. Just another thing that Mac OS X does way better than Windows (or Linux for that matter).

Now I just need to explain why this only seems to be the way of things on my system drive. On removable drives such as USB and Firewire, and can see it regenerate the thumbnails on the first pass through a folder each time I plug it in (but instantaneous there after until I unmount the drive again). I have formatted my two external drives (one Firewire, one USB2.0) for Mac OS X's native format, and so it should support "resource forks".

Any thoughts on why this only seems to be the way on my system disk?
 
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Another thought - are there any tools out there that will let you see what is in the resource fork for a file? That would bring crystal clarity to this discussion right away!
 
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It would be in hex, but HexEdit is what I use to view data and resource forks. It'll take a lot of digging, but you could probably find a piece of a PDF or PNG in a JPEG and that's most likely the thumbnail due to the fact that OSX uses PDF and PNG as the system image type.

...and I'd love to write out a description of resource forks for you, but Wikipedias already done it for me.... :)
 
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Yeah, it was a dumb question. I Googled "resource fork" right after I posted. The Wikipedia entry was the first one I read. An excellent resource (no pun intended) that Wikipedia.
 

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