What are some common limitations of backup tools? What are the limitations to SAN-based VM backups?
The vendor itself dictates the actual capabilities and limitations of given backup tools. VMware Consolidated Backup (VCB), for example, does have several limitations that a potential adopter should keep in mind before deployment. It must be able to access the storage subsystem used for virtual machine (VM) snapshots. If VCB cannot read the snapshots, it won't be able to back them up to a storage area network (SAN).
Though VCB is compatible with a variety of operating systems, it only handles file-level VM backups on Windows platforms. In addition, VCB will not back up VMs in a cluster, but the cluster will provide its own level of resilience. Neither will it back up raw device mappings when used in physical compatibility mode. If you run into roadblocks, use virtualization-aware third-party software instead of VCB and rely on backup clients to push the VM contents to a backup server.
Additional reading on SAN-based backups
Best practices to back up a SAN
Using a SAN for a centralized backup
How to move backups to a SAN
Many organizations adopt SAN-based snapshots, because SANs often include their own data protection features. This method can be particularly useful when VM files are stored on a SAN. You could even use the SAN to create a copy of a production LUN used for a VM and then use that copy on the SAN as the proxy for backups. This eliminates the need for VM-based snapshots, because the SAN provides the snapshots instead.
The principal limitation with SAN-based snapshots is scope. For example, a SAN that supports VMware Virtual Machine File System and RDM snapshots can allow both full snapshots of the whole file system and snapshots of individual VMs, which gives you greater backup flexibility. If the SAN only supports RDM, however, you can only take individual VM snapshots for backup. Take the time to evaluate the data protection capabilities of your SAN to determine the best corresponding backup options.
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