Multipart uploads used to be streamed directly to the remote with their
own PutStream machinery, bypassing the VFS, a design left over from
before the VFS could abandon a streaming write.
They are now written through the VFS exactly like plain object PUTs in
every cache mode. The parts are written, in part-number order, to a
temporary object which is renamed into place server-side on
completion.
With the default --vfs-cache-mode off the parts stream through the VFS
to the remote as they arrive. With --vfs-cache-mode writes or above
they are buffered in the VFS cache and uploaded by its write-back.
User visible changes:
- Multipart uploads now show in rclone's transfer stats and obey
--bwlimit (previously they bypassed both).
- Remotes without streaming upload support now spool the upload to a
temporary file on local disk instead of buffering it in memory.
- Multipart uploads are never buffered in memory because of missing
remote capabilities - only --disable-multipart-streaming does that.
- Remotes that upload atomically now also write to a temporary object
renamed into place, so an in-progress multipart upload is no longer
briefly visible under its final key.
- On the few remotes with no server-side move or copy the parts are
written straight to the final object in all cache modes.
- With --vfs-cache-mode writes, plain PUTs and multipart uploads to the
same key go through the same cache entry, so an earlier PUT still in
the write-back window can no longer be written back over a newer
multipart upload.
- Failed write-backs are retried by the VFS without the client having
to restart the upload, and completed objects are served from the
cache for read-after-write.
Nick Craig-Wood
b4db289d0a