Lightroom Classic's Preview file does not shrink in size when discarding Standard and 1:1 Previews

[The Keeper of the Garden]

LrC 15.1 on macOS Tahoe 26.2

Catalog contains around 98.000 RAW files

Lightroom Catalog Previews.lrdata is over 300 GB large (containing all Standard Previews of all RAW files, but almost no 1:1 previews)
Trying to reduce the file size from 300 GB to less than 200 GB by discarding the Standard Previews *and* the 1:1 Previews from almost half of my catalog's RAW files (the oldest ones) by using the command in the Library menu item (Library > Previews > Discard Standard and 1:1 Previews...) does not change the Lightroom Catalog Previews.lrdata size in any way. The file is still over 300 GB.
Restarting LrC nor Restarting the iMac helps. 
The command seems to run fine, showing the progress bar in the upper left corner of the UI window of  LrC, but fails to shrink the file size of the previews file. Thus no previews were discarded and the command does not function as it should.

[Jao vdL]

It takes a long time to actually remove the previews you discarded in my experience. I think it only marks the previews you discarded as removed and slowly cleans up in the background when it determines you actually wanted to get rid of them - i.e. you do not actually zoom in again to 1:1 to necessitate a 1:1 preview. 

[The Keeper of the Garden]

This is far from a correct answer, as it was marked as such!

Please re-read carefully my detailed, logical, and precise post. This has nothing to do with 1:1 previews but everything with *STANDARD* previews. I tried to "discard" the Standard Previews of photos (RAW files) that had absolutely no 1:1 previews, as I had not viewed/edited these old photos for years, and all of my 1:1 previews are discarded automatically by LrC after one week...
So, the bug is there and has nothing to do with the "solution" you offer.
Anyway, discarding a file should always be instantaneous. We're talking about computers and software, not about something you throw in the cellar and get rid of a year or two later. 

[Jao vdL]

The same thing is true for standard previews - sorry if I did not make that clear. The culling from the preview database for any size preview is not instantaneous. It is how they implemented it. I agree it would be preferable and logical if it were instantaneous but it clearly isn't. I have seen this on my catalogs where it will slowly decrease the preview database after you selected "discard standard and 1:1 previews". I think it won't actually discard standard previews in that case if you have recently touched or viewed the image - don't know what the time constant for that is. Also note that it will recreate standard previews in the background over time regardless if you discarded them so discarding standard previews usually has little effect. Only when you set the standard preview size to something smaller will you end up with a smaller preview database after discarding them since the recreated previews will be smaller size.

[The Keeper of the Garden]

I disagree.
1. Discarding is discarding. 
2. If this behaviour is "implemented" (I've previously heard software engineers talking about "undocumented features" when they were actually meaning "bugs...), then it makes no sense. And it doesn't correspond to what I have experienced in the last 20 years or so since I started using Lightroom in its baby days (when everything actually worked as it should...).
3. It is incorrect to state that LrC creates Standard Previews in the background, no matter what. That only happens when you actually view or edit the photo in question. Which is not what I did with the 50.000 or so photos from my early days in LrC... I haven't "touched" any of them in ten or more years.
4. Again, you still don't understand: the photos I have discarded the Standard Previews of, haven't been viewed nor edited in ten or more years. So discarding their Standard Previews should shrink the previews file with at least 100 GB in my case (considering the size of the RAW files being smaller using smaller sensor cameras in the past). 
5. Discarding should be instantaneous. Otherwise it makes no sense, as the user thinks they're gone but they're not and the software does things invisibly, arbitrarily, without any explanation or warning.

I've actually re-run the discard command on a few of my folders of old photos. Again, the progress bar showed up, making the user think it's discarding the previews, but the preview file size has remained the same. In fact, it has actually grown... 
Nothing makes sense in this way. Discarding is discarding, immediately, correctly, effectively, and visibly. This is not what happens. Therefore it's a bug or a malfunctioning command.

[Jao vdL]

I understand your frustration but the Adobe folks will probably tell you that this is as designed. I am not an Adobe engineer, just a fellow user and this is how I understand the preview database is implemented after being surprised myself I could not directly decrease the size of the database and researching the issue. Discarding doesn't delete the files instantaneously. It just marks them out of date and in the background the actual files are deleted over time and only if they were actually not needed. Standard previews are generated automatically for images in your database that don't have them if there is some request to see them which is triggered by just browsing through the folder in Classic. So while not automatic, just browsing through will kick off a regeneration of the preview.

Think of discarding in its literal meaning as casting aside or putting in a trash bin. That doesn't delete the items immediately but marks them as not needed. What your discard action does is akin to sticking a "get rid of this" post-it on the thing you're discarding. They are not actually gone until somebody permanently deletes them. There basically is a clean up service that goes through the building (your preview database) and takes out the trash (the previews you marked discarded). It can take a while before it gets to the room you're cleaning up in and even then it might decide that "hey they actually did use this after they put on the post-it so I am not yet getting rid of this quite yet".

This is what Google's AI says when comparing discard vs delete which is consistent with the dictionary meanings of these words:

"Discard" often means getting rid of something unwanted (like trash or a draft email) with a sense of personal choice, while "delete" usually implies permanent removal, especially in digital contexts (like deleting a file from a computer). In software, "discard" might hide something from a view but keep it in the database, whereas "delete" permanently removes it, though the specifics depend on the application's design (e.g., some apps use discard for your own cards, delete for anyone's). 

[johnrellis]

With my LR 15.1 / Mac OS, after doing Discard Standard And 1:1 Previews, space in the folder <catalog>Previews.lrdata isn't recovered until I then do File > Optimize Catalog.  This is analogous to deleting a file in Finder to the Trash, and then having to empty the Trash to recover the disk space.

Note that space in <catalog>.lrcat-data also isn't recovered until you do Optimize Catalog.

This behavior isn't unusual -- many databases operate this way.

[The Keeper of the Garden]

Thank you, but are you running Tahoe 26.2? Because when I decided, before I posted my OP, to try "optimising" the catalog whilst having LrC making a backup of the catalog,  the size of the Catalog Previews (.lrdata) file didn't change at all.
I can understand as you say in your comment (and also Jao above it) how databases "discard" first and "delete" afterwards, but:
1. Isn't LrC's vocabulary getting a bit complicated? There is "discard" for the previews, there is "Remove" for the photo files (with the option of "deleting" them at the same time), and there's "Delete" for Collections etc...
2. I suspect that, in the case of LrC 15.1 and macOS 26.2, when the Previews.lrdata file size isn't changing even after restarting the Mac etc, even after not "touching" the old photos of which the previews were "discarded", it could be because of an issue, once again, with File Permissions. I noticed that, when I choose to "Remove *and* Delete" one or more photos in LrC, I get the warning that because my external hard disk (where those photos reside) isn't allowed to use the "Bin", the photos have to be deleted immediately. This is a faulty assumption by LrC since I can easily "delete" (i.e. move to the Bin) any file on that external HD outside of LrC...

Anyway, I have now deleted the Previews.lrdata file of over 300 GB and will create Standard Previews whenever I feel like or have LrC create them on the fly. It's just that I want to have control over what software does with files on my Mac and don't like things to happen invisibly and arbitrarily, which in fact in my case meant that the Previews.lrdata file never shrunk a single byte, even after "discarding" half of the Standard Previews, optimising the catalog file, restarting the Mac, and waiting one or two days. I think that, just like in the case of "Removing" photos from the catalog, there should be an option added to the "Discard Standard and 1:1 Previews" command to have those previews disappear immediately, at least after a restart of LrC.

[Jao vdL]

I get the warning that because my external hard disk (where those photos reside) isn't allowed to use the "Bin", the photos have to be deleted immediately. This is a faulty assumption by LrC since I can easily "delete" (i.e. move to the Bin) any file on that external HD outside of LrC...

That's a known Mac OS problem/bug where it doesn't create a trash bin on external volumes. If the external volume doesn't have the hidden ".Trashes" folder (which is how it implements trash bins on volumes) in it's root directory, Mac OS will tell programs that want to delete a file that this is a volume that has no trash/recycle bin. The trick is to create this folder from the commandline and to make sure it has group staff assigned to it and this error message in Lightroom Classic will go away. I am pretty sure this only is a problem on non-native filesystems like exFAT or NTFS and that if you format the drive as APFS, Mac OS creates the folder by default but that is something to check I guess.

[The Keeper of the Garden]

Nope. It's not a macOS problem at all. Like I said before, any other application can easily delete files and folders on that external HD, only LrC is unable to and gives a false error message about this volume not supporting the normal method of moving the file(s) or folder(s) to the Bin when "deleting" them. That external HD is formatted as macOS Extended (Journaled), as it should be.
Previous LrC versions had the same bug/problem, and updates usually solved it. But why does it keep on coming back when they know how to fix it...?

[Jao vdL]

It really shouldn't show this error if the .Trashes folder is there and you have permission to write to that folder. I can delete images stored on external drives without any issue in Lightroom Classic and it does not show the error message. It just moves the images into the trash. The only time I have seen this is when an external drive was missing the folder. Mac OS should automatically create this hidden folder but sometimes doesn't usually if there is a permissions issue with the external drive. Permission issues arise for example if it was used in another machine with another username and you do not have "ignore ownership on this volume" checked when you do a get info on the drive.

If your externals are ssd, Apple recommends using APFS for the formatting but Mac OS extended (Journaled) is fine for spinning magnetic disks.

[The Keeper of the Garden]

Dear Jao,
I know all this... I have some 40 years of experience using computers, of which 20 years on Macs. I used to work as a software engineer and Internet specialist in a previous life.
I have a LaCie RAID system, no SSDs but good old HDs, each 8 TB in size. It was never owned or used by anyone else, never attached to another Mac. LrC has "full access" to everything in the Settings of my Mac. I assure you, this has happened before, and not only on this external HD (Raid system). All ownership settings, all permissions, everything works fine on that RAID system (which I also use when editing video in Final Cut Pro). I never encountered any problems deleting files, either manually or via other applications. Only certain versions of LrC, like the very last one, gave and give trouble, sending out warnings that aren't correct about how the Bin cannot be used. Anyway, this bug will go away eventually, whenever Adobe sends us a new version. In the mean time, I just try to ignore it, because I have backups of my photo files anyway and hardly delete any photos anyway.