Lightroom 2015.9: Why does it need to sync metadata for hundreds of photos every time changing computers?

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I'm using a two-computer setup (an iMac and a MacBook Pro) with Lightroom. LR Catalog file on a USB3 SSD (Samsung T1) together with Previews, Smart Previews and Cache folder. Every time I leave my iMac, plug the SSD to my MBP and start LR it starts to sync photos – even though nothing has changed with the photos or the LR catalog (it's just a different LR install on a different computer). Hundreds of photos start syncing for no reason. Why is this?

When I check the sync status via LR Preferences > Lightroom mobile tab Sync Activity says "Metadata" after each photo. What is this metadata that needs to be synced as there should be none as I'm running the exactly same LR catalog (just a different computer and different LR install)?

Thanks for any help. This isn't a huge issue (for now) as nothing seems to get corrupted or otherwise wonky, but it does take unnecessary time, processing power and laptop battery (as I'm on-the-go with my MBP), and it is confusing.
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anssik

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Victoria Bampton - Lightroom Queen, Champion

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It has to check what's in the cloud and cache some of that metadata locally so it knows whether it needs to sync anything. You could just pause sync.
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Jose de Barros

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jose   de barros     Portugal     braga
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anssik

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Yeah of course it needs to check for up-to-date data BUT that data is already locally up-to-date as I'm using the exactly same catalog on both computers (as the catalog is stored on a USB3 SSD, together with Smart Previews, 1:1 Previews and Cache folder).

John. R. Ellis' response below hints that there's syncing data in the user application data folder as well, which seems strange to me as LR should be storing all catalog related files on the USB3 SSD.

I don't feel comfortable pausing sync as I'm actively using LR on four different devices – iMac, iPad, iPhone and MacBook Pro – and I'd much rather have them be up-to-date always as I do make changes to photos and Collections on all of those devices.
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John R. Ellis, Champion

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I think the extra syncing is caused by where LR stores its synchronization database, in the user application data folder. You're not moving that between machines with your catalog. I'm away from my computer, but when I get back, I can send you the exact location. You could try symbolic linking from that folder to a folder on your external drive, but such hacks are always fragile.
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anssik

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Thanks, that makes sense. Such a shame if that's the case, though. I'd much rather have all sync related data staying with the catalog as that's what I'm switching between computers, not the app.
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john beardsworth

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The sync data is at :
On Mac it's at /Users//Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata
On Win it would be at C:\Users\\AppData\Local\Adobe\Lightroom\Caches\Sync Data\Sync.lrdata

You could try moving it, or deleting it each time you move over. However, I suspect this will make no difference other than giving the sync process a clean starting point.
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anssik

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Thank you for this information. I think I won't be trying to experiment with moving it to the SSD and making a Symbolic Link to it, at the moment, but would like to know whether or not this would work.

What I don't understand is why it takes so much time for just to check and sync metadata – minutes instead of seconds. I suppose this depends on how much time it's been since last using my other computer.

It's troubling as keeping LR catalog on an external USB3 SSD drive you would think would eliminate any need for syncing as it's the same catalog both computers are accessing and LR is said to keep any catalog related data on the catalog. But now I know there's catalog and sync related data on the app folders as well.

Thank you all for making this understandable.
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John R. Ellis, Champion

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"It's troubling as keeping LR catalog on an external USB3 SSD drive you would think would eliminate any need for syncing as it's the same catalog both computers are accessing and LR is said to keep any catalog related data on the catalog."

You're right, it is often said that the catalog captures all the information about your photos, and putting the catalog on an external drive is usually one of the recommended solutions for working on multiple computers.  But that advice long predates the introduction of LR Mobile, and yours is the first such report I've seen of someone using both LR Mobile and a catalog being moved to different computers.  

I don't have any educated guesses as to why Adobe decided to store synchronization state in a separate database outside of the catalog folder.   It seems unnecessary, since only one catalog can be synced with LR Mobile at any one time. But I'm sure I'm missing information about the decision.
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anssik

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Thanks for your thoughts, John! Would be nice if someone from Adobe could verify and comment on this.
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John R. Ellis, Champion

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"why it takes so much time for just to check and sync metadata – minutes instead of seconds."

My educated guess (based on having architected many application network protocols and having examined the LR sync database in detail for one of my plugins): I suspect the sync protocol is not optimized for this relatively rare use case, in which the photos reside on both the server (LR Web) and the client (LR Desktop), but the synchronization data is missing.  

In the normal use case, in which the sync data is present, most likely the sync data has some sort of change sequence number or time stamp representing the last sync, and the protocol just needs to exchange information about changes that have occurred on each side since then.   

But when the sync data is missing entirely, it has to exchange data for the entire sets of synced photos on both the server and client (as Victoria alluded), and it's probably doing this with an incremental photo-at-time messaging that's just fine for the normal case but is very slow when having to do the entire catalog.   

In addition, or alternatively, there may be sync throttling designed to ensure the syncing is being polite and not consuming too much CPU or bandwidth, and that throttling is causing the from-scratch photo-at-a-time syncing to go extra slowly.

I think it's likely that the LR Mobile architect didn't anticipate this use case.

But these are just guesses...
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anssik

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Again, thanks for great thoughts! At Adobe anyone to confirm and comment?
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steve reid

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I’m using `Lightroom mobile mainly , but syncing up to desktop Lightroom CC, and sometimes my MacBook Pro Lightroom CC.
So are trying to figure out the best way to do this . As I don’t want to have to wait 1/2 a day if I’m syncing 100GB of new shoots to when I occasionally use my laptop.
I have placed my Lightroom catalogue in a folder that gets backed up every day to a portable Hard drive , so I could copy that over to laptop to speed up the ‘sync’ and not have it have to download everything from the cloud.

I have read I could use Dropbox to keep my catalogue in that would sync between multiple computers . This is quite appealing as I am a huge Dropbox user.

Another avenue I have to explore is making a new folder and catalogue for each shoot . But on the iPad Lightroom mobile app I can’t seem to make a new catalogue on the fly or swap between catalogues .

Wouldn’t it be great if we could choose form stored catalogues in Teheran cloud?