After the Snapshot is taken, a “Map Disk Region” call is initiated by VDDK (Virtual Disk Development Kit) to obtain the mapping of disk and understand the layout of LUNs. When a backup is initiated, Commvault creates a Snapshot of the target VM. As depicted in the figure below, data is read directly from the storage LUNs where the virtual machine disks reside, without transferring data over the local area network (LAN). In case of SAN transport mode, the Virtual Server Agent (VSA) at the Commvault end must have zoning or mapping to the storage array with access to storage LUNs that provide storage for virtual machine disks. Therefore, I thought it would be good to pen down the details here, so that this blog can be useful for others.īefore we get into the details of the actual issue, let us understand what SAN transport mode is and how it works. Considerable amount of time and efforts were spent before the root cause was identified. The issue was primarily due to the SEsparse disk format and the method that SAN transport mode of the Commvault backup server adopts to perform backup of the virtual machines. One of my customer had an issue, where the Snapshot consolidation tasks were failing post backup.

Virtual disk consolidation is needed