Lightroom Image Previews folder getting too large so I moved it, Is that safe to do?

[Ken Nielsen]

My image previes folder for Lightroom was nearing one TB in size so taking up too much hard drive space, so I moved it to an external drive and now lightroom seems to know where it is as I can still see all of my previews with ratings. I have destroyed more lightroom catalogs in my life than I care to mention so I want to ask here before I trust this change. Does Lightroom automatically know where the image previews folder is and is it okay to have on an external drive? I've already deleted the original folder from the computer hard drive where all of my apps are. You would think that Lightroom would complain if it coudn't find the image preview folder. Can someone with more experience give me input on this please. How can I tell if Lightroom is actaully 'hooked' to the image preview folder in its new location and not just using some sort of 'memory' from the old folder. Lightroom takes knowledge to do things like this and as you can tell, it's 'seat of the pants' is how I've been gtetting along all of these years.

TIA,

Ken

[drtonyb · Accepted solution]

@Ken Nielsen 

The LrC previews are stored in a folder along side your catalog file.

If you moved that folder to an external drive, LrC will not 'know' where it is. However, if the previews folder no longer exists along side your catalog, because you moved it or deleted it, then LrC will recreate the previews folder and start building new previews as required.

So, no, you can't just move the previews to somewhere else. You can delete your previews folder along with all the previews in it and LrC will simply recreate the folder and rebuild the previews as you view your photos, but they will be Standard Previews. I would not build 1:1 Previews unless you really want to scrutinise a photo; it is 1:1 Previews that take a lot of space on your hard drive. You can force a 1:1 Preview build for a single photo by Zooming to 100% in Library Loupe. However, as soon as you make a change to a photo in Develop, any 1:1 Preview that exists is invalidated and when you leave Develop, Standard Previews will be created and the old 1:1 Previews will be deleted after a few minutes.

[JohanElzenga]

To build on what @drtonyb explained: ratings have nothing to do with previews. Yes, they may be shown on the preview border, but they are stored in the catalog, so moving the previews will do nothing to ratings. Lightroom Classic has simply rebuilt those previews it needed to have, that is why you still see previews and think Lightroom Classic is still using the ones you moved. It is not, however.

P.S. Why do you act first, and then come here to ask if you did the right thing? Wouldn't the opposite order make more sense?

[Ken Nielsen]

Thank You. The previews folder was approaching 1TB and taking over my hard drive forcing me to delete apps. I had to dump it as even Lightroom could no longer exist with no hard drive space.  I take it then that Lightroom regenerating previews will only need to regenerate what I am using and will remain small until someday in the next 30 years from now when it might become a monster and need to be dumped again?

[JohanElzenga]

Thank You. The previews folder was approaching 1TB and taking over my hard drive forcing me to delete apps. I had to dump it as even Lightroom could no longer exist with no hard drive space.  I take it then that Lightroom regenerating previews will only need to regenerate what I am using and will remain small until someday in the next 30 years from now when it might become a monster and need to be dumped again?

By @Ken Nielsen

Yes, Lightroom will only generate those previews it needs, so it will take time before they grow to the same size again. Plus, there are new options now in the Catalog Settings to not let that happen. You can now set a maximum size for the previews cache.

[Ken Nielsen]

Previews are important to me, I process a number of ways but full detail is what I need as I'm looking for best results and keep numerous versions processed with various external programs. Lightroom ends up being the hub of this processing and keeps all of the resulting images generated out of just one choice for each choice. That plus Photoshop processing means quite a bit of room is taken with previews. Thanks to everyone for help here. I'll just keep an eye on the folder and throw it out again next time it starts taking over the drive. Good to know that is an option and not destroying anything really as I take it it will be processed and restored if ever need be.

[Conrad_C]

In the past I threw out the previews cache when it exceeded 30 or 40GB. I had no need to keep that many previews around. There can be an argument to keep previews if you frequently browse many different folders looking for images, because the more previews are cached, the less they have to be built. But if you mostly work on recent images, using so much storage for previews of folders you rarely visit might be a waste of fast storage.

Keeping more previews might be a higher priority for older computers that take a long time to render them, and a lower priority for newer computers that can regenerate previews relatively quickly.

To address this, Lightroom Classic 14 has a new Catalog Settings option that lets you limit how large you want the previews cache to be. When the previews cache gets too large, Lightroom Classic will auto-purge excess previews. It protects the last 30 days of previews though, probably so you don’t lose previews of images you’re currently working on. I think that’s why the screen shot below says the previews are 21GB when I specified a limit of 10GB. For me, this is still an improvement over the previews file growing past 30, 40, 50GB… before this limit option was added.

It’s technically possible to use advanced techniques to make Lightroom Classic link to the previews cache at a location outside the catalog folder, such as creating a symbolic link to an external volume. However, this is considered risky and might fail, so that’s why it isn’t suggested.

It’s probably because Lightroom Classic can work with multiple catalogs that it requires the previews cache to be in the same folder as the catalog. If someone maintained a number of catalogs, and each catalog’s previews cache was located in a different location than the catalog, things could get messy fast.

This is such a common issue that maybe Adobe needs to change Lightroom Classic so that it works like the Adobe video apps, where a single cross-application Media Cache can be set by the user to be on any compatible volume (internal or external). Premiere Pro also allows various project files (e.g. rendered previews) to be stored either in the same folder as the project or at a location that you choose. But for now, with Lightroom Classic you have the options of setting that previews cache limit, moving the whole project folder to a larger volume, or continuing to throw out the previews cache manually when you think it’s gotten too big.

[Ken Nielsen]

Thank you. That is valuable insight. I'm all for keeping things safe, not throwing things into a tizzy. The option to limit preview cache size is probably all I needed to resolve this issue. I've worked with Lightroom since it was introduced and this is the first time there has been an issue with the size of the preview folder. All is well and this issue is resolved I would say.

[Ken Nielsen]

Did you arrive at allow preview cache size maximum of 10GB by trial and error or did you just pic a number that seemed like enough. I have no idea how many full size 1:1 previews it takes to make 10GB, and, like you say, newer computers can regenerate previews fairly quickly then so I may never know what would be the ideal maximum to set. I only work mostly with new projects coming in so old stuff is only referenced on a 'as needed' basis. I think that makes me an ideal candidate to set the maximum to 10GB and no longer be creating a mountain of unneeded previews. Thanks again for your help with this. I appreciate.

[Conrad_C]

10GB is my first “trial” and we’ll eventually see if that is an “error.” 🙂 It’s a laptop with limited free space, so I thought I would see how low I could stand to set it. If I get annoyed about previews not being ready enough of the time, I’ll try increasing it. If I realize I haven’t noticed the difference, I’ll leave it at 10GB.

It’s been set at 10GB for at least a couple of weeks, and I haven’t noticed much of a problem so far, so I might leave it on that value for a while longer.

[Ken Nielsen]

Peace.

[Ken Nielsen]

Thank you. I wonder if it might be good to have Adobe enable having the previews folder able to be linked to another drive since it can become so large as to take over the computer hard drive? Either way, whew, I finally have a little breathing room on my computer and can re-install my needed programs.