EMC QLogic Installation Guide download pdf (Page 139)

Understanding persistent binding

137

Troubleshooting

Understanding persistent binding

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 operating system (OS) does not provide a satisfactory

means to allow persistent binding. Most software applications access

storage using file systems managed by the Windows OS. (File

systems are represented as <drive letters> <colons>, that is, C:, D:,

and so forth.) For storage devices containing file systems, Windows

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 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 is accessed by the name \\.\PHYSICALDRIVEXXX,

where XXX is the drive number. For example, a system with three

hard disks attached using an QLogic Fibre Channel controller assigns

the disks the names \\.\PHYSICALDRIVE0,

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EMC QLogic Installation Guide Page 139