Volume Sync -
Volume sync does sync for upload volumes between what volumes should be in CS as per DB (and the user) vs what are actually present on secondary storage (it collects this info through volume.properties). If there is a parity then it issue redownload of volumes on sec. storage or syncs the state on DB if the volume is already downloaded. It helps does the sync in following way.
Upload volume is in progress and MS/SSVM shuts down, when MS/SSVM agent handshake again we sync its current state and update db accordingly for upload completed or retry as be the case.
If this doesnt happen then we might leak the volume on secondary storage. But offcourse user can always upload the same volume again.
URL expiration -