[[!comment format=mdwn
 username="https://anarc.at/openid/"
 nickname="anarcat"
 subject="thanks for considering this!"
 date="2016-09-22T12:43:11Z"
 content="""
> (Let's not discuss the behavior of copy --to when the file is not
> locally present here; there is plenty of other discussion of that in
> eg http://bugs.debian.org/671179)

Agreed, it's kind of secondary.

> git-annex's special remote API does not allow remote-to-remote
> transfers without spooling it to a file on disk first.

yeah, i noticed that when writing my own special remote.

> And it's not possible to do using rsync on either end, AFAICS.

That is correct.

> It would be possible in some other cases but this would need to be
> implemented for each type of remote as a new API call.

... and would fail for most, so there's little benefit there.

how about a socket or FIFO of some sort? i know those break a lot of
semantics (e.g. `[ -f /tmp/fifo ]` fails in bash) but they could be a
solution...

> Modern systems tend to have quite a large disk cache, so it's quite
> possible that going via a temp file on disk is not going to use a
> lot of disk IO to write and read it when the read and write occur
> fairly close together.

true. there are also in-memory files that could be used, although I
don't think this would work across different process spaces.

> The main benefit from streaming would probably be if it could run
> the download and the upload concurrently.

for me, the main benefit would be to deal with low disk space
conditions, which is quite common on my machines: i often cram the
disk almost to capacity with good stuff i want to listen to
later... git-annex allows me to freely remove stuff when i need the
space, but it often means i am close to 99% capacity on the media
drives i use.

>  But that would only be a benefit sometimes. With an asymmetric
>  connection, saturating the uplink tends to swamp downloads. Also,
>  if download is faster than upload, it would have to throttle
>  downloads (which complicates the remote API much more), or buffer
>  them to memory (which has its own complications).

that is true.

> Streaming the download to the upload would at best speed things up
> by a factor of 2. It would probably work nearly as well to upload
> the previously downloaded file while downloading the next file.

presented like that, it's true that the benefits of streaming are not
good enough to justify the complexity - the only problem is large
files and low local disk space... but maybe we can delegate that
solution to the user: \"free up at least enough space for one of those
files you want to transfer\".

[... -J magic stuff ...]

> And there is a complication with running that at the same time as eg
> git annex get of the same file. It would be surprising for get to
> succeed (because copy has already temporarily downloaded the file)
> and then have the file later get dropped. So, it seems that copy
> --from --to would need to stash the content away in a temp file
> somewhere instead of storing it in the annex proper.

My thoughts exactly: actually copying the files to the local repo
introduces all sorts of weird --numcopies nastiness and race
conditions, it seems to me.

thanks for considering this!
"""]]