Installation guide

Understanding persistent binding
185
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,