Thumbnail preview generation on scheduled task seemingly not working

Plex Media Server
1

Server Version#: 1.42.1.10060
Player Version#: 4.149.0

Hi,

i migrated my PMS about 1 month ago. (Manually, new server, adding all media as new)

As the initial scanning of my libraries took long as expected, i turned the option to generate preview only on scheduled tasks. With the days and weeks passing i noticed, that this job does not seem to run the preview generation.

I then changed the settings to generate preview on scheduled task and when new media is added, then added a new series and it startet immediatly.

image

But still the scheduled task will not generate thumbnail previews for all my already added stuff.

Is this a known bug, or am i doing something wrong.

The settings is activated per library, also in the global settings.

EDIT:
Is there a way to check, if the preview generation has been completed? I cannot imagine this task has been completed so fast.

Thank you in advance!

2

Short Answer

  • No easy way to verify video preview thumbnails have been generated for a specific media file.
  • Widening the Scheduled Tasks window may help.
  • Thumbnail generation can be manually initiated at any time via WebTools-NG.
  • You can monitor progress via Settings → Console or the Plex server log files.

Additional Information

Short of playing the media file, I do not know of an easy way to verify preview thumbnails have been generated for a specific file. The info is not visible via Media Info or XML Info.


Scheduled Tasks

Scheduled tasks run between the start/stop times configured in Settings → Scheduled Tasks. Plex may not be reaching the task to generate video preview thumbnails before the window closes (I do not know the specific order in which the tasks run). Extending the window when the tasks run, to ensure Plex has enough time to reach the thumbnail generation task, may help.


WebTools-NG

You can manually initiate preview thumbnail generation at any time via WebTools-NG - Butler Scheduled Tasks.

The “Generate Media Index Files” task will kick off video preview thumbnail generation. Plex will check all movies/tv episodes and generate thumbnails for those where they do not exist.

Download: Release 1.2.1 · WebTools-NG/WebTools-NG · GitHub
Wiki: Butler Scheduled Tasks · WebTools-NG/WebTools-NG Wiki · GitHub

WebTools does not have to run on the same system as Plex Media Server.


Monitoring Progress

You can monitor progress via Settings → Console or via the Plex server log files.

Scheduled Tasks are referred to as Butler Tasks in the Plex log files.

Video Preview Thumbnails are also referred to as Base Index Frame Files.

Log File Examples. From Plex Media Server.log & rollovers, .1.log to .5.log.

Maintenance window opens

Sep 11, 2025 03:04:39.425 [135929400232760] DEBUG - Butler: we're in the window, starting.
Sep 11, 2025 03:04:39.425 [135929400232760] DEBUG - Butler: Waking up!

Maintenance tasks finish:
Sep 11, 2025 13:35:19.075 [125276344429368] DEBUG - Butler: Done doing work.

Maintenance window closes:
Sep 11, 2025 15:01:42.411 [125276288383800] DEBUG - Butler: we're outside the window, stopping.


Here’s the log entries for generating video preview thumbnails for a tv show episode.

Beginning:

Sep 09, 2025 22:37:37.918 [130758126422840] DEBUG - [BaseIndexFrameFileManager] building index (320x240) for parts for MetadataItem 22586 (The Scarecrow Murders)
Sep 09, 2025 22:37:37.920 [130758126422840] DEBUG - [BaseIndexFrameFileManager] Activity: registered new activity 0580fc3d-f2eb-4378-bfe7-8857a40c3a51 - "Generating video preview thumbnails"
...
Sep 09, 2025 22:37:37.920 [130758126422840] DEBUG - [BaseIndexFrameFileManager/JobRunner] Jobs: Starting child process with pid 14473

Thumbnail Generation

Sep 09, 2025 22:37:38.467 [130758168435512] DEBUG - [Req#2f64b/Transcode/bif/0580fc3d-f2eb-4378-bfe7-8857a40c3a51] Activity: updated activity 0580fc3d-f2eb-4378-bfe7-8857a40c3a51 - completed 0.0% - Generating video preview thumbnails
.....
Sep 09, 2025 22:43:04.148 [130758165711672] DEBUG - [Req#302da/Transcode/bif/0580fc3d-f2eb-4378-bfe7-8857a40c3a51] Activity: updated activity 0580fc3d-f2eb-4378-bfe7-8857a40c3a51 - completed 100.0% - Generating video preview thumbnails

Generation Complete

Sep 09, 2025 22:43:04.161 [130758251354936] DEBUG - Jobs: '/usr/lib/plexmediaserver/Plex Transcoder' exit code for process 14473 is 0 (success)
Sep 09, 2025 22:43:04.339 [130758126422840] DEBUG - [BaseIndexFrameFileManager] Activity: Ended activity 0580fc3d-f2eb-4378-bfe7-8857a40c3a51.
3

Yes it is.
If the text “Indexes: SD” is present, there are thumbnails.

grafik
grafik908Ă—643 29.8 KB

It appears very similarly in the XML, as indexes="sd".

1 Like
4

@OttoKerner Thanks!

5

@towerplease

With the info from @OttoKerner, there is a way to determine if video preview thumbnails have been generated.

You can use WebTools-NG ExportTools to export information about your media from the Plex database into a CSV file. This includes if previews have been generated. You can then import the info into a spreadsheet to make viewing easier.

Here’s an example with a custom export with the media thumbnail generation.

Custom Export - a custom export titled “Video Preview Thumbnails.”

  • Title = Media Title
  • Part Indexed = If thumbnails have been generated (Indexes in Media Info)
  • Part File Combined = the directory path to the file

custom level
custom level1515Ă—1144 87.1 KB



Export Tools - Note Export Level = Video Preview Thumbnails
export
export1516Ă—1152 81.4 KB



Spreadsheet
CSV info imported into a Google Sheets spreadsheet.
spreadsheet
spreadsheet1986Ă—279 61.5 KB

6

Thank you for this information.

As i guessed, only the newly added media files (with option to create video preview thumbnails when files are added) are indexed. All previously added media dont have “index sd”.

So it really seems, that the scheduled job isnt working. Any chance to start that process manually?

7

Thank you for that. I will have a look and see.

8

Hi again,

i took an export as you suggested, and as you can see, almost all files have N/A for Index:

image
image1742Ă—218 25.1 KB

Now the question… Is the scheduled job bugged for indexing or can i manually start that process?

9

I’ve the same PMS version on Ubuntu and it generated the thumbnails during scheduled tasks overnight. I haven’t noticed any other QNAP posts, but there could still be something QNAP specific.

Check the server logs to see if thumbnail generation starts during the scheduled tasks window. See my first reply for details. If you upload a set of logs I’ll take a look over the weekend.

Try expanding the scheduled tasks window. Set scheduled tasks to start at 0000 and end at 2300 (basically run all day). Then check progress occasionally over the next couple of days.

You can manually initiate thumbnail generation via WebTools Butler Tasks → Generate Media Index Files (see first reply). It doesn’t necessarily help with problem diagnosis (unless it fails), but it will generate the thumbnails.

10

I checked the logs (see Attached)

Plex Media Server Logs_2025-09-13_20-57-00.zip (4.6 MB)

I can find many entries regarding “butler” but nothing to do with Thumbnail generation.

It looks like its running, but not even trying to generate..

Thank you very much in advance!

EDIT:
I started “Generate Media index Files” from WebTools-NG. But nothing happened in Plex Dashboard. I also started a Database Backup, which appeared within 10 seconds.
EDIT2:
Ok, now after around 5 Minutes, the Video Preview Generation started visibly in Dashboard :slight_smile:
EDIT3:
The Video Preview Generation finished in like 20 Minutes. The Job didnt ran through the whole library :frowning:

11

Hello again.

After a couple of days, i can see the scheduled task is likely to work now, as i see the creation of thumbnails.

Unclear is, wether this is now triggered due to the extended schedule windows (00:00 - 23:00) or if there is any other issue, which prevents this scheduled task into running correctly in my previous set windows (23:00 - 07:00)

I will let this process run for a the nexts weeks and see if all media is processed correctly.

Thanks!

12

Same here. I activated the generation for thumbnails a while ago and manually triggered it for every library. I waited and tried a view things, but there are still no thumbnails for my old collection. Just the newly added stuff. How can I trigger it or is this a real bug?