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How intelligent switches can improve virtualization

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In-band, out-of-band and SPAID
In a virtualized SAN fabric, there are three ways to deliver applications: in-band, out-of-band or split path architecture for intelligent devices (SPAID). To understand the advantages and disadvantages of each approach, you need to be familiar with how the metadata server, control path software and data path software operate in those three architectures:

Metadata server. This server maintains the configuration database for the storage services provided. For virtualization services, this database contains the entire mapping between virtual volumes and physical devices.

Control path software. This software provides the interface between the metadata server and the data path software. It also performs background I/O tasks for applications, such as copying data from a snapshot to a remote location (data replication), resynchronizing a broken/restored mirror (virtualization) and third-party copy functions (backup).

Data path software. This moves the data from servers to storage and vice versa. It also performs the actual translation from virtual to physical addresses using mapping tables passed to it from the metadata server.

In-band and out-of-band describe where the metadata server and the control path software reside in the network. It's important to understand that in all cases, the data path software resides in the data path.

In the case of in-band, the metadata management, the control path processing and the data path processing are all performed by the same computing elements. In other words, all three are "in the path." For out-of-band implementations, the metadata management and the control path processing are both performed by a separate compute engine other than the data path software. Given that the majority (more than 95%) of the transfers are data transfers, a fast and efficient data path results in excellent performance and better scalability.

In-band products have been typically represented by Intel server-based appliances such as IBM's SVC, DataCore and FalconStor where the appliance provides all the computing power. These products are relatively simple to deploy, but suffer from performance bottlenecks and scalability issues as workloads increase. This is why these appliances have not made serious inroads in enterprise environments.

Out-of-band solutions, represented by StoreAge's SAN management applications, run the metadata server and the control path software in an Intel server appliance connected to the FC SAN and deploy data path software as an agent on each of the application servers. This results in excellent transfer speed and scalability. The biggest negative for out-of-band solutions is the need to place agents in each host. These agents are operating system- and platform-specific, making them impractical for many enterprise environments.

This was first published in October 2004

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