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I'm repeatedly seeing this message in nemo, despite clicking Fix now each time. I'd like to know if there's something I can do to fix the thumbnail cache once and for all.

A bug is filed for when the thumbnail directories are symlinked to /tmp, but this isn't the case for me. However, I do have various symlinks in, out and within my ~/Pictures directory structure.

I've found references to nemo --fix-cache, but no docs (man nemo doesn't mention this option). It's occurred to me to try something like sudo find -type d -exec nemo --fix-cache {} ;, but I'm wary of this being an undocumented feature, and I don't want to stuff things up even more.

Does anybody have any experiences to share?

I'm running Nemo 2.6.7 under Mint 17.2.

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2 Answers 2

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Some information that might makes it easier to track the issue.

--fix-cache

The --fix-cache option was included in version 2.6.2. It is not in man but found by:

nemo --help
...
--fix-cache    Repair the user thumbnail cache - this can be useful if
               you're having trouble with file thumbnails.
               Must be run as root

From changelog:

[ Michael Webster ]
* Adapt Nemo to deal with problem situations with the user thumbnail cache: 
  - Added --fix-cache command line option (must be run as root) 
  - Added quick check at startup to detect major problems and disable
    thumbnailing until fixed. 
  - Individual thumbnailed files are checked on the fly for permission issues. 
  - When a problem is detected, an infobar appears, offering to fix the problem,
    or ignore it.
* thumbnails: Remove an orphan comment from previous commits, 
              fix wording of the --fix-cache command-line option.

ThumbnailFactory

Nemo uses the GnomeDesktopThumbnailFactory.

As such it creates a thumbnail in a cache directory. Typically ~/.thumbnails/[SIZE] or ~/.cache/thumbnails/[SIZE]. Where depends on system environment; more specifically xdg settings like XDG_CACHE_HOME. N.B! This is a shared cache that can be used by multiple applications.

The file name of a thumbnail is generated by hashing the original file's URI. The library can then check if the file is already cached by looking at the hash sum of the it's location.

If an error occurs a file is created in .../thumbnails/fail/gnome-thumbnail-factory/[HASH].png

Identify source for cached thumbnail

ThumbnailFactory add PNG tEXt properties to the thumbnails. Among these are the URI. You can use various tools to extract this information, for example with Imagemagick:

identify -format "%[Thumb::URI]\n" ~/.cache/thumbnails/normal/774b4a5ba74455xxx.png
file:///home/foo/some_image.jpg

or the more verbose:

identify -format "%[*]" ~/.cache/thumbnails/normal/774b4a5ba74455xxx.png

or simply:

identify -verbose ~/.cache/thumbnails/normal/774b4a5ba74455xxx.png

This information is also stored in the failed thumbnails.

Custom thumbnail generator

One can add custom thumbnail generators by adding configuration files in share/thumbnailers. Typically: /usr/share/thumbnailers/ or ~/.local/share/thumbnailers

Example taken from arch Linux PCManFM for creating PDF thumbnails with Imagemagick:

[Thumbnailer Entry]
TryExec=convert
Exec=convert %i[0] -thumbnail %s %o
MimeType=application/pdf;application/x-pdf;image/pdf;

You could try this as a last resort. (Believe) nemo uses GTK library for thumbnail generation of images.

You might

  • Try to run the --fix-cache as root.
  • Try to remove, (or move), the thumbnails cache directories – start fresh. N.B: The cache directory is shared between applications using GnomeDesktopThumbnailFactory.
  • Run nemo from command line to see if any specific errors are reported.
  • ...
2

I'm facing this as of 2017 with Linux Mint 18.1 (Serena), so I think it merits an updated answer. This problem got registered both as Github issue #1429 and Launchpad bug #1592293.

The fix came with this commit, which got tagged to version 3.4.2 of Nemo, so that should be the released version that fixed the issue.

The solution to this problem is then to just get an updated version of Nemo. Updating to the latest release of Linux Mint (18.2) should be enough; however its Release Notes don't specify which is the exact version of Nemo included.

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