[[!comment format=mdwn
 username="joey"
 subject="""comment 2"""
 date="2020-02-28T18:58:48Z"
 content="""
Export remotes don't use chunks, and use CHECKPRESENTEXPORT rather than
CHECKPRESENT. And git-annex tries to not buffer the whole worktree in
memory, but stream through it, so it can support very large worktrees.
So this idea, which I do think is a good idea, seems limited to checking 
chunks.

(I'd probably want to make the chunk handling code only include up to 1
million or so chunk keys in a request, again to avoid using too much memory.
1 million chunk keys needs 160 mb ram max, 80 or less typically.)

At least for purposes of chunks, the reply to CHECKPRESENT-MULTI only needs
to say if all the keys are present. If even one chunk is missing, the object
as a whole is not present in the remote. That seems like a useful simplication.

Internally, probably Remote.checkPresent should change to taking a `[Key]`
list. Simpler than adding a whole other method for this.

Remote.External could use CHECKPRESENT when there's one key in the list,
and CHECKPRESENT-MULTI when the are multiple, falling back to CHECKPRESENT
on an UNSUPPORTED-REQUEST reply. But, I think it ought to be an
extension to the protocol, to avoid that extra roundtrip.
"""]]