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Move away from the LUN for storage snapshots

LUNs, or logical unit numbers, are used to provision storage in virtual environments. Creating LUNs is also a process that many storage administrators might call the bane of their existence. Howard Marks, chief scientist at DeepStorage.net, has some strong words about LUNs as well. "The LUN, the logical volume as a unit of abstraction, must die. Its time has come and gone," he said.

Marks told the audience of his presentation at TechTarget's Storage Decisions conference that it is time to manage workloads on a per-virtual machine (VM). The virtual machine, he said, exists for application purposes, not just storage purposes such as the LUN. With more integrated and clustered workloads in virtual environments, the LUN as a unit of abstraction no longer serves a purpose. 

"I think it's very important that we move from the LUN to the VM for providing data services," Marks said. This is particularly true when working with backup features such as snapshots. Working on a per-VM basis is far more productive than taking a snapshot of a LUN, which may contain just one useful VM in a large pool of data. The goal is to provide application-consistent, quiesced snapshots. For him, this does not include vSphere snapshots. "To use a deeply technical term, vSphere snapshots 'doth sucketh,'" Marks said, citing the increased latency and decreased IOPS when taking a log-based vSphere snapshot.

Marks explained that in the past, copy-on-write was sufficient, but the lengthy and complicated procedure sacrificed performance. "You have to read data from the old location, you have to copy it to the new location, and then you have to write the new data to the original location," he said. However, in a modern storage system, snapshots are handled as a metadata operation, just like clones.

When managing storage snapshots through metadata, creating a snapshot of a VM means the metadata that defines that VM is copied. This creates a clone on which new data can be written. "That [new data] gets written to a new block location, and the metadata for that clone gets updated, and the metadata for the parent gets left alone," Marks said.

This results in a much lower impact on performance, and the snapshots can help with both data protection and backup. "And because it's all happening in metadata, we can clone VMs very quickly."

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