I just uploaded the whole picture collection of my wife to our nextcloud instance, including documentation photos for her work, all together >100 GB. The uploading itself via desktop client took several days of partly active laptop and one night at last to fully finish. The preview generator was set to pre-generate every 30 minutes and I guess was running though out the whole upload time until now, ~30 hours after the upload was finished.

This is on a Raspberry Pi 2 btw with data folder on USB hdd dock with 3 TB WD Red inside, so weak hardware overall ;).

Of course this is an exceptional situation. On important production systems one should stop the pre-generate cronjob during upload and then choose some calm hours to do generate-all, breaking it during active times. But of course this strongly depends on hardware, active hours and on how (which apps in what way) the nextcloud instance and other software on the server is used. In the end sometimes you give the previews simply high priority because you want to share them soon with others, being able to smoothly watch them with gallery app. Sometimes it doesn't matter too much. I would also not recommend to implement too much checks and therefore run the cron job the one or the other way. I think there are just too much parameters and cases that count. Of course some sort of web ui or occ command options for [max.] process time, files/time etc. would be nice to start the pre-generation/generate-all command with fitting settings for such individual cases. But in the end e.g. I had no idea how long the pre-generation of the whole collection would take and still will and how much the usage of the server/nextcloud instance could be effected. So even some nice options could have been chosen wrong.

Another way to solve this would be to some how make the process pause/stop by e.g. web ui access, the standard nextcloud cron.php, file accesses etc., go on again ~10 minutes after the last access or just start again with the next cron execution. Maybe this can be achieved moreless by giving the process a very low priority on hardware usage?

Also I asked myself if/how it is checked, if preview generation is already running. Found this in execution command:

			if (($this->time->getTime() - $lastActivity) < 30 * 60) {
				$output->writeln('Command is already running.');
				return 2;
			}

Does this mean, that the generation just starts, if it's last activity is 30 minutes ago? Therefore the readme suggestion "I run it every 10 minutes" would not make too much sense, or did I get something wrong?

Use a background job (instead of cron) #1

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