During rollback we query the remote oplog, fetching only a couple of small fields from each oplog entry, and we return up to 16 MB per batch, or about 600 k entries. This requires reading up to 600 k entire oplog entries on the remote end, and if the oplog entries are large and not in cache this can be a very substantial amount of data to be read from disk (tens or hundreds of GB), and may require more than the hard-coded 10-minute timeout to complete. In this case the rollback times out and cannot complete.