74 lines
2.9 KiB
Text
74 lines
2.9 KiB
Text
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Currently, the git-annex assistant syncs with remotes in a way that is
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dumb, and potentially inneficient:
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1. Files are transferred to each reachable remote whose
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[[preferred_content]] setting indicates it wants the file.
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2. After each file transfer (upload or download), a git sync
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is done to all the remotes, to update location log information.
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## unncessary transfers
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There are network toplogies where #1 is massively inneficient.
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For example:
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<pre>
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laptopA-----laptopB-----laptopC
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\ | /
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\---cloud based repo--/
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</pre>
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When laptopA has a new file, it will first send it to laptopB. It will then
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check if the cloud based transfer repository wants a copy. It will, because
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laptopC has not yet gotten a copy. So laptopA will proceed with a slow
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upload to the cloud, while meanwhile laptopB is sending the file over fast
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LAN to laptopC.
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(The more common case with no laptopC happens to work efficiently.
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So does the case where laptopA is paired with laptopC.)
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## unncessary syncing
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Less importantly, the constant git syncing after each transfer is rather a
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lot of work, and prevents collecting multiple presence changes to the git-annex
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branch into larger commits, which would save disk space over time.
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In many cases, this sync is necessary. For example, when a file is uploaded
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to a transfer remote, the location change needs to be synced out so that
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other clients know to grab it.
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Or, when downloading a file from a drive, the sync lets other locally
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paired repositories know we got it, so they can download it from us.
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OTOH, this is also a case where a sync is sometimes unnecessary, since
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if we're going to upload the file to them after getting it, the sync
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only perhaps lets them start downloading it before our transfer queue
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reaches a point where we'd upload it.
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It would be good to find a way to detect when syncing is not immediately
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necessary, and defer it.
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## mapping
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Mapping the repository network has the potential to get git-annex the
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information it needs to avoid unnecessary transfers and/or unncessary
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syncing.
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Mapping the network can reuse code in `git annex map`. Once the map is
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built, we want to find paths through the network that reach all nodes
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eventually, with the least cost. This is a minimum spanning tree problem,
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except with a directed graph, so really a Arborescence problem.
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A significant problem in mapping is that nodes are mobile, they can move
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between networks over time. This breaks LAN based paths through the
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network. Mapping would need a way to detect this. Note that individual
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git-annex assistants can tell when they've switched networks by using the
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`networkConnectedNotifier`.
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## P2P signaling
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Another approach that might help with these problems is if git-annex
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repositories have a non-git out of band signaling mechanism. This could,
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for example, be used by laptopB to tell laptopA that it's trying to send
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a file directly to laptopC. laptopA could then defer the upload to the
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cloud for a while.
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