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» Enterprise IT Planet » Storage » Storage Features

Virtualization, Storage Closely Linked

By Chris Evans
December 10, 2010

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Of the many areas of IT infrastructure affected by virtualization, storage is clearly near the top. Storage expert Chris Evan overviews the technology.

Virtualization, as it continues to gain adoption, has a very close relationship with storage. Virtual machines are just data and have to be stored somewhere. On standalone virtual servers, this can be achieved simply by using DAS (locally attached) disks. But if more advanced virtualization features (and increased availability) are required then virtual machines will normally be stored on SAN or NAS arrays. Using a dedicated storage array provides a number of significant benefits to the user, namely:

• Increased resilience - storage arrays operate with a high degree of redundancy with multiple power supplies, fans and other internal components.

• Scalability - storage arrays are highly scalable devices and can be extended dynamically without outages.

• Shared access - storage arrays allow shared access to data, which isn't generally practical or possible with DAS. Shared access is essential in increasing resiliency.

• Increased functionality - storage arrays offer advanced features such as replication and snapshots that can offload processing power from the virtual server.

Virtual guests all contained on a single LUR; LUN failover keeps replicated LUNS together.

Storage Features Key to Virtualization


As virtual servers increase in levels of adoption, they will encompass more and more mission critical systems. It will be essential, therefore, for virtual guests to be replicated between locations to improve availability and protect in the event of a disaster scenario at the local data center.

Replication can be performed synchronously (where I/O is confirmed as written to both source and target arrays before being confirmed to the host), or asynchronously (where the source array confirms I/O complete to the host without waiting to confirm the target array has received the data).

Storage arrays are highly efficient at replicating data, which has been a key feature of these devices for 15-20 years. However, for SAN arrays, the way in which data is presented to the host can cause an issue with replication.

Storage is typically presented to virtual servers as large LUNs (Logical Unit Number). This is a single unit of storage as far as the array is concerned, but from the virtual server perspective it will be used to hold many virtual guests.

For example, a 500GB LUN could hold 25 virtual guests of 20GB per guest. Each of these virtual guests will have their own service levels and DR requirements. The storage array will replicate the entire LUN and in the event of a “failover” scenario where operation moves to the remote array, it is expected that the primary LUN will not be accessed, and all host I/O will occur on the remote LUN.

This may be fine for a complete DR failover but doesn’t address other operational requirements, for example, where a single VM guest is moved to another location for operational reasons rather than a DR outage.

Read the rest at Datamation.

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