--batch combined with -J now runs batch requests concurrently for many
commands. Before, the combination was accepted, but did not enable
concurrency. Since the output of batch requests can be in any order, --json
with the new "input" field is recommended to be used, to determine which
batch request each response corresponds to.
If --json is not used, batch mode still runs concurrently, using the usual
concurrent-output. That will not be very useful for most batch mode users,
probably, but who knows.
If a program was using --batch -J before, and was parsing non-json output,
this could break it. But, it was relying on git-annex not supporting
concurrency despite it being enabled, so it should have expected concurrent
output. So, I think that's ok.
annex.jobs does not enable concurrency in --batch mode, because that would
confuse programs that use --batch but don't expect concurrency.
git-annex allows managing files with git, without checking the file
contents into git. While that may seem paradoxical, it is useful when
dealing with files larger than git can currently easily handle, whether due
to limitations in memory, checksumming time, or disk space.
For documentation, see doc/ or <https://git-annex.branchable.com/>
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