Having very big files in chunked special remotes makes operations like fsck, move or drop extremely slow. Eg. a movie of 20GiB in a special remote with the recommended chunk size of 50MiB results in 400 lookups each time the presence of the key is checked. One could increase the chunk size, but that has other disadvantages: a) Chunks are apparently still buffered in memory b) one would lose more transfer progress if the special remote does not implement resumable transfers and c) it would render the use of git annex inprogress for streaming purposes useless. Letting the remote know about all the keys that are about to be checked would allow it to optimise the request to the remote server.

So I propose a protocol extension CHECKPRESENT-MULTI Key1 Key2... to let the remote do the optimised lookup after which it can reply with a list of present keys PRESENT Key1 Key2.... This list can be empty if none of the keys are present: PRESENT.