jrk

Bootstrapping Folder Sync With Existing Data

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Is it possible (and safe) to bootstrap a large folder sync via outside channels (like sneakernet)?

 

Concretely:

 

- Large folder exists on A

- Folder is shared from A to B

- Sync starts, trickles along

- Sync agent is stopped on B

- Original folder data is copied via backchannel from A to B

- Sync agent is re-started on B

 

It's easy to imagine that B's sync process will simply reindex the contents of the folder upon restarting, discover that everything is identical (or very nearly so), and quickly complete without sending much more of the original contents over the network. But it's also easy to imagine that this could tickle subtle bugs or strange behavior by being outside the expected interactions with a partially-synced directory.

 

Is this type of setup safe? Have other people done this?

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That type of setup is indeed safe, due to the fact that you're dealing with identical data.

What will happen is that both devices will continue to index. As they index, they will exchange information on the files they've indexed. Those files will then be checked against each other and the one with the newer modified timestamp will be used.

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The one difficulty with this is that if you delete a bunch of files on the "master" copy while the sneakernet flash disk is still on its way to the destination, these files will show up again. This is because there is no way to run the initial sync in any mode other than "merge".

Another scenario where specifying one copy as a master would help: you wipe your laptop's software and reinstall everything. Your data partition is still there, with a copy of the synced folders that is now out of date. You cannot realistically just install BTSync and add that folder, because again, this will obliterate deletions that happened in the meantime. The best option is to wipe and start from scratch, unless you know the two to definitely be in sync.

Here's a suggestion I've made about this a long time ago: http://forum.bittorrent.com/topic/16462-re-sync-previously-synced-folders/?p=42364

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