Multi-path I/O, serial-attached SCSI meet high performance storage demands

Rapidly shrinking form factors and rising performance expectations are placing a new set of challenges in front of storage system designers. Just as small, dense blade servers are driving the deployment of smaller form factor disk drives, the limitations of existing bus architectures and parallel interfaces are making it increasingly difficult for designers to match drive performance to capacity.

The combination of two new technologies, Multi-Path I/O (MPIO) computer configurations and serial-attached SCSI (SAS) interface technology, offer system designers a new opportunity to meet escalating performance, reliability, scalability and manageability requirements.

Combining SAS and MPIO offers designers a variety of benefits. First, it offers a much lower cost structure than comparable Fibre Channel alternatives since SAS hardware costs remain as low or lower than legacy SCSI hardware while using mature SCSI software.

Also, SAS hardware offers designers more flexible cabling and device addressability options. SAS devices use two different types of expanders, called edge and fan-out expanders, to extend the connection capability of a SAS host adapter board. Fan-out expanders connect up to 128 edge expander devices and an edge expander device set supports up to 128 physical links. In addition, one SAS initiator could support up to 16,383 devices on a single SAS transmission bus and support a cable length of up to eight meters without any equalization.

Dual-port SAS disk drives add physical link redundancy function to SAS target devices. Designers can implement a dual-port disk drive as either a single target device or two target devices sharing the same logical unit.

By combining SAS and MPIO, a designer can support multiple initiators and targets in the same SAS domain. Moreover, designers can use SAS technology to implement MPIO system in a rack, cluster-in-a-rack, or cluster-in-a-room configurations.

Using a SAS/MPIO configuration, designers can connect a server to a SAS RAID storage system through multiple and independent I/O paths. Storage system designers can also construct MPIO configurations which use SAS expanders. The designer can make the configuration behind SAS RAID controller invisible to the MPIO configuration. To provide complete fault-tolerance, however, the SAS storage system must provide RAID configuration and function.

Since SAS devices support hot-plug functionality, the controller firmware is responsible for reporting device status change and new device discovery. This allows system designers to give system administrators the capability to receive updated device status, modify the MPIO pathways and configure the automatic load-balancing control on the storage system to meet specific system applications.

Software designers can provide a graphical user interface which allows system administrators to maximize system performance and path efficiency by balancing the I/O workload across multiple MPIO paths. Designers can use SAS controller firmware and drivers to report data pathway failures and events, and any errors to the operating system. Through this monitoring and reporting, the storage system can automatically operate the path load-balancing and failure recovery functions.

Fit for mid-range

Two example SAS/MPIO implementations offer designers attractive options for mid-range storage system designs using a single server with MPIO and a two-node computer cluster. The first configuration features a single MPIO server with more than one available PCI slot, two dual-ported SAS RAID controllers, two SAS edge expanders and three or more dual-port SAS disk drives.

This configuration provides multi-path I/O fail-over, fail-back and load-balancing functions. The SAS pathway from RAID controller port to disk drive port determines the MPIO Path ID. If the server is as a database server and an email server, it can fully use the I/O bandwidth with both active paths. This MPIO configuration can add new SAS expanders or disk drives without shutting down the server or the storage system.

A second example configuration combines an MPIO cluster with SAS storage devices. It requires two servers, two SAS controllers, and one SAS RAID Storage system with dual controllers.

This two-node cluster with SAS/MPIO supports multi-path I/O fail-over, fail-back, load-balancing functions and server service fail-over and fail-back functions. If the server functions as a database server and an email server, it also can fully use the I/O bandwidth with both active paths and both active cluster nodes.

Designers can implement this two-node cluster with MPIO in a cluster-in-box or cluster-in-a-rack format.

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