EMC QLA23xx, QLA22xx manual Understanding Persistent Binding in a Fabric Environment

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Introduction

Understanding Persistent Binding in a Fabric Environment

Persistent binding is the mechanism to create a continuous logical route from a storage device object in the Windows host to a volume in the EMC® storage array across the fabric.

Without a persistent binding mechanism, the host cannot maintain persistent logical routing of the communication from a storage device object across the fabric to an EMC storage array volume. If the physical configuration of the switch is changed (for example, the cable is swapped or the host is rebooted), the logical route becomes inconsistent, causing possible data corruption if the user application is modifying data through inconsistent logical routing of the communication from the driver entry point to a volume in an EMC storage array across the fabric.

The Windows NT/Windows 2000/Windows 2003 operating system (OS) does not provide a satisfactory means to allow persistent binding. Most software applications access storage using file systems that are managed by the Windows OS. (File systems are represented by drive letters: colons: C:, D:, etc.) For storage devices containing file systems, Windows NT/Windows 2000 writes a Disk Signature to the disk device. The operating system can then identify, and associate with, a particular drive letter and file system.

Since the signature resides on the disk device, changes can occur on the storage end (a cable swap, for example) that can cause a disk device to be visible to the host server in a new location. However, the OS looks for the disk signature and, providing that nothing on the disk changed, associate the signature with the correct drive letter and file system. This mechanism is strictly an operating system feature and is not influenced by the Fibre Channel device driver.

Some software applications, however, do not use the Windows file systems or drive letters for their storage requirements. Instead they access storage drives directly, using their own built-in “file systems.” Devices that are accessed in this way are referred to as raw devices and are known as physical drives in Windows terminology.

The naming convention for physical drives is simple and is always the same for software applications using them. A raw device under Windows NT/Windows 2000/Windows 2003 is accessed by the name \\PHYSICALDRIVEXXX, where XXX is the drive number. For example, a system with three hard disks attached using a QLogic Fibre Channel controller assigns the disks the names

1-2 EMC Fibre Channel with QLogic HBAs in Windows Hosts