High Availability Solutions

Keeping mission-critical data continuously available has become a requirement over a wider range of customer segments from small business to datacenter environments. Enterprise environments using Windows Server require 99.999 percent uptime for all key workloads including file server, database, messaging and other line of business applications. This level of availability can be difficult and very costly to achieve and it requires that redundancy be built in at multiple levels: storage redundancy, backups to separate recovery servers, server clustering, and redundancy of the physical path components.

Application Availability through Failover Clustering

Clustering is the use of multiple servers, Host Bus Adapters (HBA’s), and storage devices that work together to provide users with high application availability. If a server experiences a hardware failure, or is temporarily unavailable, end users are still able to transparently access data or

applications on a redundant cluster node. In addition to providing redundancy at the server level, Clustering can also be used as a tool to minimize the downtime required for patch management and hardware maintenance. Clustering solutions require software that enables transparent failover between systems. Windows Server Failover Clustering (WSFC) or Microsoft Cluster Server (MSCS) as it was previously known is one such solution that is included with the Enterprise Edition and Datacenter versions of Microsoft Windows Server 2008 and Windows Server 2003.

Storage Redundancy through RAID

High availability solutions based on redundant arrays of independent disks (RAID) have been in use with mainframe equipment for several decades, and have been supported in the Windows

operating system since the first release of Windows NT. RAID solutions provide protection of data through the use of redundant disks, which can be configured for both fault tolerance and/or improved performance.

Storage Availability through MPIO

The Microsoft MPIO framework allows Windows to manage and efficiently use up to 32 paths from storage devices and applications/operating systems. Although both MPIO and WSFC result in high availability and improved performance, they are not equivalent concepts. While clustering provides high application availability and tolerance of server failure, MPIO provides fault tolerant connectivity to storage. By employing MPIO and WSFC together as complimentary technologies users are able to mitigate the risk of a system outage at both the hardware and application levels.

Note: When using the iSCSI Software Boot initiator, a maximum of 2 paths to the boot volume are supported.

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MPIO provides the logical facility for routing I/O over redundant hardware paths connecting server to storage. These redundant hardware paths are made up of components such as the cabling, Host Bus Adapters (HBA’s), Switches, and Storage Controllers and possibly even power. MPIO solutions logically manage these redundant connections so that I/O requests can be rerouted in the event that a component along one path fails.

As more and more data is consolidated on Storage Area Networks (SAN’s), the potential loss of access to storage resources is unacceptable. To mitigate this risk, high availability solutions, like MPIO, have now become a requirement.

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MPIO Solutions

MPIO is a Microsoft-provided framework that allows storage providers to develop multipath

solutions that contain the hardware specific information needed to optimize connectivity with their storage arrays. These modules are called Device Specific Modules (DSM’s). The concepts around DSM’s are explored in greater detail later on within this document.

The Microsoft MPIO solution is protocol independent and can be used with FC, iSCSI, parallel SCSI, and Serial Attached SCSI (SAS) interfaces on the following operating systems

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Windows Server 2008 Windows Server 2003

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Multipath solutions in Windows Server 2008

With Windows Server 2008, an IT storage administrator has several of choices while deploying a MPIO solution. These include: ? ?

Deploy the Microsoft MPIO framework and use a storage vendor provided DSM for Windows Server 2008 in a FC, iSCSI, or SAS shared storage configuration

Deploy the Microsoft MPIO framework using the Microsoft DSM (MSDSM) which is a generic DSM provided for Windows Server 2008 in a FC, iSCSI, or SAS shared storage configuration

The notable enhancements to the Microsoft MPIO solution in Windows Server 2008 include:

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As previously discussed, a complete solution that an IT department can deploy along with the appropriate hardware

Support for Asymmetrical Logical Unit Access (ALUA) controller model as defined in SCSI Primary Commands-3 (SPC-3).

Support for storage arrays that follow the Active/Active controller model. This support allows for using all available I/O paths when they are healthy.

Note: When using the MSDSM, storage that implements an Active/Active storage scheme but does not support ALUA will default to use the “Failover-Only” Load Balance policy, although a different policy may be chosen later.

Multipathing solutions are not eligible to receive logo qualification unless they adhere to the MPIO framework. IT administrators should verify the logo status of array vendor solutions. The following link lists solutions logoed with vendor storage products.

For additional information on logo requirements please refer to the WinQual Logo Site (https://winqual.microsoft.com/default.aspx)

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Failover Clustering in Windows Server 2008 does not support the use of parallel SCSI storage

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With Windows Server 2003, an IT storage administrator can:

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Deploy a solution available only from vendors for a SAN. The storage array partner builds their DSM and offers the solution using the Microsoft MPIO framework that Microsoft

makes available to partners. On the Windows Server 2003 platform, the partner provides a Device Specific Module (DSM) that understands the nuances of the vendor hardware and

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plugs into the Microsoft provided MPIO architecture.

Deploy the Microsoft MPIO DSM for iSCSI included in the Microsoft iSCSI Software Initiator package

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This joint solution allows storage partners to design hardware solutions that are integrated with the Windows operating system. Compatibility with both the operating system and other partner provided storage devices is ensured through the Windows Logo program tests to help ensure proper storage device functionality. This ensures a highly available multipath solution using Microsoft MPIO which offers supportability across Windows OS implementations.

Customers should refer to guidance from their hardware storage array partner on which DSM to use with their storage. Multipath solutions are supported as long as a DSM is implemented in line with Logo requirements for MPIO. Most Multipath solutions for Windows today do use the MPIO architecture and use a DSM provided by the storage IHV. Customers may use the Microsoft DSM provided by Microsoft in Server 2008 if supported by storage partner as well. Customers should refer to their storage array partner for guidance on which DSM to use with a given storage array as well as the optimal configuration.

Multipath software suites available from storage IHV’s typically provide an additional value add

beyond the implementation of the MSDSM as the software typically also provides auto configuration, heuristics for specific storage arrays, as well as statistical analysis, and integrated management, It’s generally recommended to use the DSM provided by the hardware storage array partner to achieve optimal performance since the storage array partner can make more advanced path decisions in their DSM specific to their array.

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On Microsoft Windows Server 2003, the storage provider will redistribute the MPIO framework with the storage provider’s DSM.

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