I have been trying to setup two computers running LRC with the same images and catalogue. I only work at one computer at a time and travel between the houses were the computers are. I use Chrono Sync over internet to synchronise the images and catalogue to the computers local drive. In most cases the time working on one computer to changing to the next will be no less then 12 hours but the syncing is often failing and the sync time is over 21 hours! I find that it is the "Lightroom master-v13-3 Previews.lrdata" that is taking extremely long to finish. My catalogue is 1.25 GB and usually I add or delete just a few MB of images. Is my thinking wrong here? Do I need to sync the preview files every time?
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Sync Chronosync is taking too long to backup my LrC files to a different computer
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You can simply rebuild the previews...
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When deleted "{catalog name} Previews.lrdata". will automatically get created new and empty by LrC . You can choose to rebuild manually. The size is limited by the Cache setting in the Previews tab of Catalog Settings.
The folder "{catalog name} Smart Previews.lrdata" is only needed if you have your Master image file on an EHD and are using a laptop where you might unmount the EHD. It is unnecessary in other cases and should (IMO) be deleted and the "Build Smart Previews" check box in the Import Dialog left unchecked.
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There is a file which holds blobs of filter data that needs to be preserved. I am not sure what it is offhand... This file also needs to be included with your lrcat file if doing backups outside the standard Lightroom backup routine.
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It is called "{catalog name}.lrcat-data"
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In that situation I would exclude the previews and keep the others.
In a network sync situation, there are two problems with the …previews.lrdata file. One is that it can be huge (many gigabytes), which takes up time on its own, especially if the Internet connection’s upload speed is not great. The other, less commonly known reason is that it isn’t a single file, but a collection of much smaller files. On the Mac it’s in macOS “package” format (you can right-click it and choose Show Package Contents), and on Windows I think it’s a folder of files. When I run Show Package Contents on my previews.lrdata file, I see it contains over 100,000 small files. This is because each image that has a preview built might be represented by several files: a 1:1 preview, Standard preview, thumbnail preview for the Grid, etc.
This extremely large number of files means there is a tremendous amount of overhead involved in transferring the whole thing, which slows down the transfer even more. It always takes more time to manage the transfer of the same amount as many small files instead of a few large files, even locally. It's like having 40 people to move between locations, and the difference between being provided one 40-seat bus or 10 cars. The 10-car solution needs 9 more drivers, a lot more parking space, and more coordination.
Excluding the previews means each computer needs to generate and maintain its own previews, but I think that’s a smaller price to pay than when trying to sync previews over the Internet. I doubt it’s a problem with ChronoSync, this would probably happen with any sync software.
In a network sync situation, there are two problems with the …previews.lrdata file. One is that it can be huge (many gigabytes), which takes up time on its own, especially if the Internet connection’s upload speed is not great. The other, less commonly known reason is that it isn’t a single file, but a collection of much smaller files. On the Mac it’s in macOS “package” format (you can right-click it and choose Show Package Contents), and on Windows I think it’s a folder of files. When I run Show Package Contents on my previews.lrdata file, I see it contains over 100,000 small files. This is because each image that has a preview built might be represented by several files: a 1:1 preview, Standard preview, thumbnail preview for the Grid, etc.
This extremely large number of files means there is a tremendous amount of overhead involved in transferring the whole thing, which slows down the transfer even more. It always takes more time to manage the transfer of the same amount as many small files instead of a few large files, even locally. It's like having 40 people to move between locations, and the difference between being provided one 40-seat bus or 10 cars. The 10-car solution needs 9 more drivers, a lot more parking space, and more coordination.
Excluding the previews means each computer needs to generate and maintain its own previews, but I think that’s a smaller price to pay than when trying to sync previews over the Internet. I doubt it’s a problem with ChronoSync, this would probably happen with any sync software.