How are unique message blob filenames allocated?

Quick question.

Are blob filenames allocated sequentially based on what is in the message store directory already or by a counter somewhere?

Background:
I have moved 7.2.0 from an old to a new server in three phases,
1) Move all of /opt/zimbra when old mail server is still running
2) Stop old server and rsync everything except the message store
3) Start new server and run rsync for the blob store to catch up

This is to reduce downtime when both servers have to be offline.

The concern is that new mail arriving before the blob store rsync in step (3) is complete may be re-allocated blobs already allocated between step (1) and (2).

OK. Solved it.

The answer was on this wiki page.

Part of the filename is based on a message ID in the MySQL database that is a "KEY" value, which basically means it autoincrements each time a new entry is created. This guarantees that file in the blob store will definitely have a unique file name. So there is no danger in migrating the store directory in two passes.