Is having the smart previews file, lrdata, mandatory?

No you don't have to have Smart Previews.  That's why their creation is optional.

And yes, you can re-create both Smart and Standard previews at a later date.  Smart Previews will not be automatically rebuilt, however.

My opinions:

Yes, you need to leave the .LRDATA. While Lightroom will recreate it if it is gone, by removing it, you are slowing yourself and your computer down, so it is not "hogging" disk space.

If the disk space on your computer is so tight that the .LRDATA is a problem, then the solution is to get more disk space. The solution is not getting rid of .LRDATA.

Indeed as DJ states, you shouldn't delete the standard "Previews.lrdata" folder (I assumed you were only talking about the "Smart Previews.lrdata" folder) . 

If you think the former is  taking up too much space and can't get a bigger drive then set the size a bit smaller but if you set it too small you will, as DJ says, find yourself waiting for LR to rebuild previews.

I would not delete any .lrdata files, as dj said LR will recreate them and I would expect LR to do so.

As for "Smart Previews.lrdata" hogging space, it will only hog space if the OP uses smart previews, I don't use smart previews (at least yet) and don't have a "Smart Previews.lrdata" file, I only have "Catalog Previews.lrdata" file which I would never delete.

I am assuming that if the OP has a "Smart Previews.lrdata" file, the OP must be using Smart Previews, perhaps because of LR Mobile.

Just for reference, widely quoted in this forum - Preference and other file locations in Lightroom 5

Whereas I assume the OP has created Smart Previews without originally understanding what they were for...hence their question (which I read as being simply a request for confirmation). 

Could very well be the case, but the OP hasn't giving us enough information to establish why Smart Preview are created. It would be helpful, if the OP would clarify further.

The simple answer to the OP is that Lightroom will only make use of "Smart Previews" in the event the original imported files are not available. So if the original files are always available then the generation of smart previews are a waste of time and drive space.

Agree but could you expand what do you mean with:

DdeGannes wrote:

 

Lightroom will only make use of "Smart Previews" in the event the original imported files are not available.

I can only think of an scenario where this would be the case, using LR Mobile. How else would the original files not be available?

How else would the original files not be available?

You want to do some editing in Lightroom on your laptop, but you don't want to bring along the (large) original photos which would take up a lot of space. You have your catalog and smart previews and so you can do all the editing you want, without the original photos. This has nothing to do with Lightroom Mobile.

One other scenario is if the original files are on an external drive that isn't connected or even on a different PC......that's how I use Smart Previews....

I can travel with my full catalog and Smart Previews on a Laptop.  The Master Files on their various disks remain at home....been doing this since Smart Previews came along, much more powerful than LR Mobile.

@Modesto Vega.

You may wish to work with a Laptop and have your original raw files stored on an external drive connected via usb or network. This is fine while you are at your home location.

If you wish to visit a friend or client and have access to the files for working in the develop module you can create "Smart Previews" for a batch of files and store the previews on your internal drive.

You are now to have greater flexibility to move around without having to transport your full Raid Stack of Drives.

The only problem with trying to work with smart previews on the same computer where the master files are located, but with the master files not being available, is that you don't have access to external editing like from Photoshop or other external editors. I got caught with that one day. I don't work with a laptop, and do all my editing on a single computer. So I choose not to use smart previews. They provide no benefit that I can see.

I agree Jim, as a retired individual I work at home on my desktop so all these features do not interest me. I mostly share my images via the Publish function to Flickr and Facebook.

Fully aware of all of this. Just to summarise, probably for my own benefit, hopefully for the OPs, this thread.

- Smart Previews are best used if the files that are not physically kept with the catalog - e.g., external hard drives    

- Smart Previews are also used by LR Desktop and LR Mobile to communicate with each other

- If, the files are physically kept with the catalog then there is no much use for Smart Previews, but LR does not stop you from creating them

Is this a fair summary?

Quote "but LR does not stop you from creating them"

I am not sure what you are inferring here but Lightroom does not create smart previews by default. The user has to either specifically select the option in the import dialog or from the Library Menu bar.

The user makes the decision.

- Smart Previews are best used if the files that are not physically kept with the catalog - e.g., external hard drives  

If the external HD is plugged in all the time, as mine are, there is no need for smart previews.

If the external HD is sometimes unplugged, then yes smart previews will be helpful

- If, the files are physically kept with the catalog there is no much use for Smart Previews

This is kind of vague, as to the meaning of "files are physically kept with the catalog". Files on a network server and catalog on the internal HD of your computer does not strike me as the case where the "files are physically kept with the catalog", yet smart previews are not required here

@subirnag210:

Yes you could remove some files from the catalog but I wouldn't take such a drastic action.

If you want a smaller lrdata file then just changing your previews settings may accomplish that....

Go to:   Edit, Catalog Settings, File Handling. 

What is your current setting for "Automatically discard 1:1 Previews"?

If it's set at "Never" then that's the reason for your large lrdata file. 

I have mine set at "30 days" but you can have it set even smaller.  This way the Previews are only kept for the files you've most recently worked upon.  If you revisit an old file the Preview will be recreated.

dj_paige wrote:

- If, the files are physically kept with the catalog there is no much use for Smart Previews

This is kind of vague, as to the meaning of "files are physically kept with the catalog". Files on a network server and catalog on the internal HD of your computer does not strike me as the case where the "files are physically kept with the catalog", yet smart previews are not required here

You are right about this. Now this is a setup I have never managed to get to work.

But this is might very well be the subject for another thread not to overload the OP.