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How does moving huge VMDK files affect storage?

VMware ESXi 5.5 enabled VMDK files larger than 2 TB, but huge VMs bring storage challenges.

Does moving to huge VMDK files have any effect on my storage subsystem, snapshots or other data protection?

As enterprise workloads grow larger and more sophisticated, the VMDKs created for those virtualized workloads also grow to sustain them. It's a simple matter to grow a VMDK file up to a conventional limit of 2 TB, and VMDKs can often be expanded on the fly. But moving beyond the 2 TB limit has been problematic, usually demanding a mix of high-end storage hardware and third-party software, and often sacrificing multiple virtualization features in the process. The release of ESXi 5.5 allows IT administrators to create VMDK files up to 64 TB and accommodate VM growth into the foreseeable future.

As VM files grow to 2 TB and beyond, there are inevitable implications for storage-related practices, including snapshots, backups, replication and migration. The biggest direct impact is time; as VMDK files grow, it takes longer to perform storage tasks or maintenance activities (like checking disk integrity). For example, even when only incremental snapshots are used, snapshots of huge VMs can potentially be much larger. This increases the time required to perform snapshots and the storage capacity needed to retain them. These time considerations will affect snapshot frequency and retention decisions. The same is true for simple data protection, where it can take far longer to backup or restore huge VMDK files, or replicating the protected data to a remote location may take significantly longer, or will require additional network bandwidth.

Even local workload migration from one server to another, like workload cloning and vMotion tasks, can take much longer with huge VMDK files. However, shared storage (such as a storage area network) can typically mitigate such migration-related delays because there is no need to move the files in shared storage architectures. But additional caution is needed when moving huge VMDK files between hosts or clusters, because the selected destination system must also meet the requirements (such as ESXi 5.5 and VMFS-5) needed to support VMDK files larger than 2 TB. If the destination server does not meet minimum requirements, or uses differing storage technologies to support larger volumes, VMDK files may not transfer properly.

Finally, adopting huge VMDK files should underscore the vital importance of thin provisioning in a virtualized enterprise to mitigate storage costs. Thin provisioning allows IT administrators to create a logical storage volume, but need only to provision enough physical storage to accommodate the workload's immediate needs. More storage can easily be allocated later as the workload's needs grow. Thin provisioning demands careful attention and management of storage utilization, but can forestall massive storage expenditures.

This was last published in January 2015

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