Ceph

What would such an API look like? Copy is implemented using read_iterate... we could add a user-supplied callback that reports the byte offset.

Whatever it is, we should do the same for import, export, and any other long-running operation.

I would say the callback route would be best.

Something along the lines of:

callback(int totalChunks, int chunks, int chunkSize, void* userData)

would be good.

For being more in line with the read_iterate() that'll call it, I suggest:

for C:

int callback(uint64_t total_len, uint64_t ofs, const char *chunk_data, uint64_t chunk_len, void *user_data);

for C++:

class RbdCopyContext {
 public:
   int callback(uint64_t total_len, uint64_t ofs, bufferlist& bl) = 0;
 };

Works for me

Thanks, Yehuda. I suppose it does make sense to provide a C++-style API with a virtual base class, just to be consistent with the rest of the interface. As Sage said, we should also make this slightly more generic and call it something like ProgressContext / callback so that it can be used with some other rbd operations.