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Preview Generator

Nextcloud app that allows admins to pre-generate previews. The app listens to edit events and stores this information. Once a cron job is triggered it will start preview generation. This means that you can better utilize your system by pre-generating previews when your system is normally idle and thus putting less load on your machine when the requests are actually served.

The app does not replace on demand preview generation so if a preview is requested before it is pre-generated it will still be shown.

How to install

  • Install directly from within your Nextcloud from the app store
  • Clone this repository in you Nextcloud app folder

How to use the app

  1. Install the app
  2. Enable the app
  3. Run ./occ preview:generate-all once after installation.
  4. Add a (system) cron job for ./occ preview:pre-generate
  • I run it every 10 minutes

Known issues

  • The app does not work with encryption enabled

How does the app work

  1. Listen to events that a file has been written or modified and store it in the database
  2. On cron run request previews for the files that have been written or modified

If a preview already exists at step 2 then requesting it is really cheap. If not it will be generated. Depending on the sizes of the files and the hardware you are running on the time this takes can vary.


preview:generate-all [--path=PATH ...] [user_id ...]

Loop over all files and try to generate previews for them. If one or multiple user ids are supplied it will just loop over the files of those users. You can also limit the generation to one or more paths using --path="/[username]/files/[folder path]", e.g. --path="/alice/files/Photos". Note that all given user_ids are ignored if at least one path is specified.


Do the actual pregeneration. This means only for new or modified files (since the app was enabled or the last pregeneration was done).


Why can't I use webcron?

Preview generation can be a very long running job. Thus we need a system that does not time out.

I get "Command already running"

Yes this happens when the pre-generate command crashes. No worries the lock will be released after 30 minutes of inactivity from the app. So go grab a cookie.

I don't want to generate all the preview sizes

This is possible since version 1.0.8. Just set the correct values via the command line

./occ config:app:set --value="32 64 1024"  previewgenerator squareSizes
./occ config:app:set --value="64 128 1024" previewgenerator widthSizes
./occ config:app:set --value="64 256 1024" previewgenerator heightSizes

This will only generate:

  • square previews of: 32x32, 64x64 and 1024x1024
  • aspect ratio previews with a width of: 64, 128 and 1024
  • aspect ratio previews with a height of: 64, 256 and 1024


  • preview sizes are always a power of 2.
  • The smallest size is 32
  • The max size is determined by your preview settings in config.php

I get "PHP Fatal error: Allowed memory size of X bytes exhausted"

You need to increase the memory allowance of PHP, by default it is 128 MB. You do that by changing the memory_limit in the php.ini file.

If you use the docker container you need set the environment variable PHP_MEMORY_LIMIT instead.

I want to skip a folder and everything in/under it

Add an empty file with the name .nomedia in the folder you wish to skip. All files and subfolders of the folder containing .nomedia will also be skipped.

I want to reset/regenerate all previews

WARNING: This is not supported but it has been confirmed to work by multiple users. Proceed at your own risk. Always keep backups around.

  1. Remove the folder your-nextcloud-data-directory/appdata_*/preview
  2. Optional: change parameters preview_max_x and preview_max_y in config.php (e.g., to 512), and change the previewgenerator app parameters heightSizes, squareSizes and widthSizes as per the README (or better yet, to a low value each, e.g. 512, 256 and 512 respectively)
  3. Run occ files:scan-app-data (this will reset generated previews in the database)
  4. Run occ preview:generate-all [user-id] (this will run very fast if you did step 2)