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.
Disable threshold -
Please do note that a cluster is chosen for vm allocation only when it meets the criteria of (used capacity of cluster + reserved capacity of cluster + requested capacity for vm / (total capacity of cluster * over provisioning)) < disable threshold. And so it can be the case that individual hosts utilization is over the threshold. The formula is applied both both for RAM and CPU. Default value of threshold = 0.85. The function of disable threshold is that capacity allowed for allocation shouldn't exceed the threshold.
URL expiration -