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Understanding Persistent Binding in a Fabric Environment
1-3
Introduction
\\PHYSICALDRIVE0, \\PHYSICALDRIVE1, and
\\PHYSICALDRIVE2. The number is assigned during the disk
discovery part of the Windows boot process.
During boot-up, the Windows OS loads the driver for the storage
HBAs. Once loaded, the OS performs a SCSI Inquiry command to get
information about all of the attached storage devices. Each disk drive
that it discovers is assigned a number in a semi-biased first come, first
serve fashion based on HBA. (Semi-biased means that the Windows
system always begins with the controller in the lowest-numbered PCI
slot where a storage controller resides. Once the driver for the storage
controller is loaded, the OS selects the adapter in the
lowest-numbered PCI slot to begin the drive discovery process.)
It is this naming convention and the process by which drives are
discovered that makes persistent binding (by definition) impossible
for Windows NT/Windows 2000/Windows 2003. Persistent binding
requires a continuous logical route from a storage device object in the
Windows host to a volume in an EMC storage array across the fabric.
As mentioned above, each disk drive is assigned a number in a first
come, first serve basis. This is where faults can occur.
Example Imagine this scenario—A host system contains controllers in slots 0,
1, and 2. Someone removes a cable from the QLogic controller in host
PCI slot 0, then reboots the host.
During reboot, the Windows OS loads the QLogic driver during
reboot and begins disk discovery. Under the scenario presented
above, there are no devices discovered on controller 0, so the OS
moves to the controller in slot 1 and begins naming the disks it finds,
starting with \\PHYSICALDRIVE0. Any software applications that
were accessing \\PHSYICALDRIVE0 before the reboot will be
unable to locate their data on the device, because it has changed.
The following figure shows the original configuration before the
reboot. HBA0 is in PCI slot 0 of the Windows host. Each HBA has
four disk devices connected to it, so Windows has assigned the name
\\PHYSICALDRIVE0 to the first disk on HBA0. Each disk after that
is assigned a number in sequence as shown in the figure.