preview

On fly preview generation can consume a lot of memory and trigger kernel's oom killer. As workaround install Preview Generator app.

Install the app
Configure the app (see below).
Enable the app
Run ./occ preview:generate-all once after installation.
Add a (system) cron job for ./occ preview:pre-generate. I run it every 10 minutes.

Generated Previews are stored in data/appdata_$instanceid/preview/

recommended configuration

From: Understanding and improving Nextcloud Previews

occ config:app:set previewgenerator squareSizes --value="32 256"
occ config:app:set previewgenerator widthSizes  --value="256 384"
occ config:app:set previewgenerator heightSizes --value="256"
occ config:system:set preview_max_x --value 1024
occ config:system:set preview_max_y --value 1024
occ config:system:set jpeg_quality --value 60
occ config:app:set preview jpeg_quality --value="60"