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DAS boot

One of the key reasons for DAS’s continued popularity in the data center is the need for a local boot drive. While most SANs support some form of booting methodology, it still requires specialized host bus adapters (HBAs) and specific support on the SAN storage system. As a result, most physical servers still boot from DAS storage.

Thanks to solid-state drives (SSDs), booting from the local server offers some specific advantages over booting from the SAN. First, servers can now be booted or re-booted in seconds from a local SSD. And the SSD can be used as a virtual memory paging area, which is incredibly important in virtual environments. As hosts in these environments get loaded up with virtual machines (VMs), they can quickly run out of RAM and begin to use local storage as a memory paging area. If this local storage is hard disk, performance can degrade substantially. When this local storage is memory based, like flash SSD, the drop in performance is negligible. SSD as a boot drive allows for more virtual machines without the need to purchase expensive RAM.

Extending the SAN with DAS

Solid-state storage also plays another role in the resurgence of DAS adoption: as an extension to the SAN. Leveraging even higher performing PCIe-based solid-state storage, architectures are now developing that allow the tiering or caching of data directly to the server needing it. PCIe SSDs can communicate directly with the CPU and don’t

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get bogged down by SAS or SATA protocols like typical SSDs. This again makes an ideal virtual memory paging area for RAM-constrained systems, but it’s the tiering or caching use case that’s becoming increasingly interesting.

With this architecture, storage systems can intelligently pre-stage the most active data within the PCIe SSD. Then, when a request for data is made by an application or user, it will be available for high-speed delivery on the PCIe SSD. This means the application or user doesn’t have to wait for the request to travel across the storage network, be accepted and processed by the storage controllers, wait for hard drives to rotate into position and then send the requested data or write acknowledgment all the way back up that infrastructure.

If successful, this model of storage architecture design would turn the SAN world upside down. Storage on the SAN would become the central repository of information that’s growing cold and the local PCIe SSD DAS would be used for the most active data. The SAN would be used for long-term retention or backup, and the server would be used for active processing. This would lead to SAN storage system designs where capacity is the focus and performance is less important. But the one downside to native PCIe SSDs is that you can’t boot from them, so a local SAS hard drive or even an SSD in a drive form factor would still be required.

This was first published in May 2012

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