=== Preview generation: ===

  * in config.php

    enable_previews -> true / false
    preview providers -> check for certain type

  * storage location:

    data/appdata_instanceid/preview

  * previewgenerator app 
<code bash>
    # generates previews to preview storage location
    # using config settings
    occ preview:cleanup
    occ preview:pre-generate # -> only new files!
    occ preview:generate-all -vvv --workers=n username  # use a worker count of ~ 2*cpu cores
    occ files:scan-app-data
</code>
== Metadata storage: ==

  * pre-v33: ''oc_filecache'' table

  * from v33: ''oc_previews'' table


=== Fix: ===

  - config.php : 'enable_previews' => false,
  - ''occ preview:cleanup''
  - ''DELETE FROM oc_filecache WHERE path LIKE 'appdata_<instanceid>/preview/%';''
  - ''occ files:scan-app-data''
  - config.php : 'enable_previews' => true,
  
=== Regenerate previews if lost or broken: ===

  - disable ALL nc cron jobs
  - stop cachefilesd (faster and not writing a lot of disk in vain)
  - run fix 
  - log in with nc user: e.g. w_nc (don't sudo, not working)
  - ''nc_directory # php occ preview:generate-all -vvv --workers=n username''
  - re-activate cron-jobs
  - start cachefilesd