286 lines
11 KiB
Markdown
286 lines
11 KiB
Markdown
This is a summary todo covering several subprojects, which would extend
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git-annex to be able to use proxies which sit in front of a cluster of
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repositories.
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1. [[design/passthrough_proxy]]
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2. [[design/p2p_protocol_over_http]]
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3. [[design/balanced_preferred_content]]
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4. [[todo/track_free_space_in_repos_via_git-annex_branch]]
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5. [[todo/proving_preferred_content_behavior]]
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## table of contents
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[[!toc ]]
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## planned schedule
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Joey has received funding to work on this.
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Planned schedule of work:
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* June: git-annex proxies and clusters
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* July: p2p protocol over http
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* August, part 1: git-annex proxy support for exporttree
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* August, part 2: [[track_free_space_in_repos_via_git-annex_branch]]
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* September, part 1: balanced preferred content
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* September, part 2: streaming through proxy to special remotes (especially S3)
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* October, part 1: streaming through proxy continued
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* October, part 2: proving behavior of balanced preferred content with proxies
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[[!tag projects/openneuro]]
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## work notes
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* Concurrency issues with RepoSizes calculation and balanced content:
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* What if 2 concurrent threads are considering sending two different
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keys to a repo at the same time. It can hold either but not both.
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It should avoid sending both in this situation.
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* There can also be a race with 2 concurrent threads where one just
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finished sending to a repo, but has not yet updated the location log.
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So the other one won't see an updated repo size.
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The fact that location log changes happen in CommandCleanup makes
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this difficult to fix.
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Could provisionally update Annex.reposizes before starting to send a
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key, and roll it back if the send fails. But then Logs.Location
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would update Annex.reposizes redundantly. So would need to remember
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the provisional update was made until that is called.... But what if it
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is never called for some reason?
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Also, in a race between two threads at the checking preferred content
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stage, neither would have started sending yet, and so both would think
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it was ok for them to.
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This race only really matters when the repo becomes full,
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then the second thread will fail to send because it's full. Or will
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send more than the configured maxsize. Still this would be good to
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fix.
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* If all the above thread concurrency problems are fixed, separate
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processes will still have concurrency problems. One case where that is
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bad is a cluster accessed via ssh. Each connection to the cluster is
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a separate process. So each will be unaware of changes made by others.
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When `git-annex copy --to cluster -Jn` is used, this makes a single
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command behave non-ideally, the same as the thread concurrency
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problems.
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* `fullybalanced=foo:2` can get stuck in suboptimal situations. Eg,
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when 2 out of 3 repositories are full, and the 3rd is mostly empty,
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it is no longer possible to add new files to 2 repositories.
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Moving some files from one of the full repositories to the empty one
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would improve things, but is there any way for fullybalanced to know
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when it makes sense to do that?
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If this is not resolved, it may be better to lose the ":number" part
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of balanced and fullybalanced. With 1 copy balanced, this situation does
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not occur. Users wanting 2 copies can have 2 groups which are each
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balanced, although that would mean more repositories on more drives.
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Size based rebalancing may offer a solution; see design.
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* "fullybalanced=foo:2" is not currently actually implemented!
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* `git-annex info` in the limitedcalc path in cachedAllRepoData
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double-counts redundant information from the journal due to using
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overLocationLogs. In the other path it does not, and this should be fixed
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for consistency and correctness.
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* implement size-based balancing, so all balanced repositories are around
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the same percent full, either as the default or as another preferred
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content expression.
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## completed items for August's work on balanced preferred content
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* Balanced preferred content basic implementation, including --rebalance
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option.
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* Implemented [[track_free_space_in_repos_via_git-annex_branch]]
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## completed items for August's work on git-annex proxy support for exporttre
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* Special remotes configured with exporttree=yes annexobjects=yes
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can store objects in .git/annex/objects, as well as an exported tree.
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* Support proxying to special remotes configured with
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exporttree=yes annexobjects=yes.
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* post-retrieve: When proxying is enabled for an exporttree=yes
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special remote and the configured remote.name.annex-tracking-branch
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is received, the tree is exported to the special remote.
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* When getting from a P2P HTTP remote, prompt for credentials when
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required, instead of failing.
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* Prevent `updateproxy` and `updatecluster` from adding
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an exporttree=yes special remote that does not have
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annexobjects=yes, to avoid foot shooting.
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* Implement `git-annex export treeish --to=foo --from=bar`, which
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gets from bar as needed to send to foo. Make post-retrieve use
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`--to=r --from=r` to handle the multiple files case.
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## items deferred until later for p2p protocol over http
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* `git-annex p2phttp` should support serving several repositories at the same
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time (not as proxied remotes), so that eg, every git-annex repository
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on a server can be served on the same port.
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* Support proxying to git remotes that use annex+http urls. This needs a
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translation from P2P protocol to servant-client to P2P protocol.
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* Should be possible to use a git-remote-annex annex::$uuid url as
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remote.foo.url with remote.foo.annexUrl using annex+http, and so
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not need a separate web server to serve the git repository. Doesn't work
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currently because git-remote-annex urls only support special remotes.
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It would need a new form of git-remote-annex url, eg:
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annex::$uuid?annex+http://example.com/git-annex/
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* `git-annex p2phttp` could support systemd socket activation. This would
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allow making a systemd unit that listens on port 80.
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## completed items for July's work on p2p protocol over http
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* HTTP P2P protocol design [[design/p2p_protocol_over_http]].
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* addressed [[doc/todo/P2P_locking_connection_drop_safety]]
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* implemented server and client for HTTP P2P protocol
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* added git-annex p2phttp command to serve HTTP P2P protocol
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* Make git-annex p2phttp support https.
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* Allow using annex+http urls in remote.name.annexUrl
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* Make http server support proxying.
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* Make http server support serving a cluster.
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## items deferred until later for [[design/passthrough_proxy]]
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* Check annex.diskreserve when proxying for special remotes
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to avoid the proxy's disk filling up with the temporary object file
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cached there.
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* Resuming an interrupted download from proxied special remote makes the proxy
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re-download the whole content. It could instead keep some of the
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object files around when the client does not send SUCCESS. This would
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use more disk, but without streaming, proxying a special remote already
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needs some disk. And it could minimize to eg, the last 2 or so.
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The design doc has some more thoughts about this.
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* Streaming download from proxied special remotes. See design.
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(Planned for September)
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* When an upload to a cluster is distributed to multiple special remotes,
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a temporary file is written for each one, which may even happen in
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parallel. This is a lot of extra work and may use excess disk space.
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It should be possible to only write a single temp file.
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(With streaming this won't be an issue.)
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* Indirect uploads when proxying for special remote
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(to be considered). See design.
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* Getting a key from a cluster currently picks from amoung
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the lowest cost remotes at random. This could be smarter,
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eg prefer to avoid using remotes that are doing other transfers at the
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same time.
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* The cost of a proxied node that is accessed via an intermediate gateway
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is currently the same as a node accessed via the cluster gateway.
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To fix this, there needs to be some way to tell how many hops through
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gateways it takes to get to a node. Currently the only way is to
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guess based on number of dashes in the node name, which is not satisfying.
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Even counting hops is not very satisfying, one cluster gateway could
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be much more expensive to traverse than another one.
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If seriously tackling this, it might be worth making enough information
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available to use spanning tree protocol for routing inside clusters.
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* Optimise proxy speed. See design for ideas.
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* Speed: A proxy to a local git repository spawns git-annex-shell
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to communicate with it. It would be more efficient to operate
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directly on the Remote. Especially when transferring content to/from it.
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But: When a cluster has several nodes that are local git repositories,
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and is sending data to all of them, this would need an alternate
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interface than `storeKey`, which supports streaming, of chunks
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of a ByteString.
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* Use `sendfile()` to avoid data copying overhead when
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`receiveBytes` is being fed right into `sendBytes`.
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Library to use:
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<https://hackage.haskell.org/package/hsyscall-0.4/docs/System-Syscall.html>
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* Support using a proxy when its url is a P2P address.
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(Eg tor-annex remotes.)
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## completed items for June's work on [[design/passthrough_proxy]]:
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* UUID discovery via git-annex branch. Add a log file listing UUIDs
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accessible via proxy UUIDs. It also will contain the names
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of the remotes that the proxy is a proxy for,
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from the perspective of the proxy. (done)
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* Add `git-annex updateproxy` command (done)
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* Remote instantiation for proxies. (done)
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* Implement git-annex-shell proxying to git remotes. (done)
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* Proxy should update location tracking information for proxied remotes,
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so it is available to other users who sync with it. (done)
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* Implement `git-annex initcluster` and `git-annex updatecluster` commands (done)
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* Implement cluster UUID insertation on location log load, and removal
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on location log store. (done)
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* Omit cluster UUIDs when constructing drop proofs, since lockcontent will
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always fail on a cluster. (done)
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* Don't count cluster UUID as a copy in numcopies checking etc. (done)
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* Tab complete proxied remotes and clusters in eg --from option. (done)
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* Getting a key from a cluster should proxy from one of the nodes that has
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it. (done)
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* Implement upload with fanout to multiple cluster nodes and reporting back
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additional UUIDs over P2P protocol. (done)
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* Implement cluster drops, trying to remove from all nodes, and returning
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which UUIDs it was dropped from. (done)
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* `git-annex testremote` works against proxied remote and cluster. (done)
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* Avoid `git-annex sync --content` etc from operating on cluster nodes by
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default since syncing with a cluster implicitly syncs with its nodes. (done)
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* On upload to cluster, send to nodes where its preferred content, and not
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to other nodes. (done)
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* Support annex.jobs for clusters. (done)
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* Add `git-annex extendcluster` command and extend `git-annex updatecluster`
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to support clusters with multiple gateways. (done)
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* Support proxying for a remote that is proxied by another gateway of
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a cluster. (done)
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* Support distributed clusters: Make a proxy for a cluster repeat
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protocol messages on to any remotes that have the same UUID as
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the cluster. Needs extension to P2P protocol to avoid cycles.
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(done)
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* Proxied cluster nodes should have slightly higher cost than the cluster
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gateway. (done)
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* Basic support for proxying special remotes. (But not exporttree=yes ones
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yet.) (done)
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* Tab complete remotes in all relevant commands (done)
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* Display cluster and proxy information in git-annex info (done)
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