copy cmd proposal

[John Berthels]

Hi folks,

I'd like to add a copy_file cmd to mogilefs (assuming there isn't one
already?).

While it's possible with a read/write loop at the moment, it would be
possible for a mogile worker to be more efficient and either:

- if src has > 1 replica, steal and DAV MOVE a replica, then force
replicate on src & dst to get back to replication policy

- if src has only one replica and device holding it permits, use DAV
COPY to create another (I/O is local to one storage server), then
force replicate

- fall back to read/write loop otherwise

this would be a potentially long-lived operation, so I thought it
could be implemented as a new 'start_copy_file' protocol cmd.


Client API could be:

MogileFS::Client::copy_file($src_domain, $src_key, $dst_domain,
$dst_key, {timeout => how_long_to_wait});

with implementation along these lines:

do_request cmd_start_copy_file
loop until timeout (fail) or get_paths (dst_domain, dst_key) returns a path

Or maybe it makes sense to expose start_file_copy directly, and let
the app do the polling loop. Or have it be an optional argument.

It may also make sense to have the cmd_start_file_copy fail if it
can't do the DAV MOVE or DAV COPY, leaving it to the client to
fallback to the read/write loop in that case.


Does anyone have a better approach than this? Is it worth my while
coding up an implementation of this for review, or is it badly flawed
in some way?

regards,

jb