HP StoreEver Tape Libraries Failover User Guide (5697-3323, June 2014)

Figure 15 Driver and system organization
Storage bus filter driver
The storage bus filter driver is named HPtapeFailover_filt and manages the OS notifications
indicating that devices have been added or removed. The storage bus filter driver bus enumerator
is usually "PCI". It attaches as an upper filter to all HBA drivers. It then monitors the creation of
raw devices, looking for supported failover capable devices. Requests to all other devices are
passed through unchanged. When a supported device is recognized, the hardware ID of the
device is changed to a value that the upper device driver will recognize, but that the rest of the
system will not. The special hardware ID prevents the standard tape class driver from attaching to
the raw paths.
Multi-path intermediate class driver
The intermediate class (IC) driver, HPtapeFailover_mpio, has two halves. The upper half is a
virtual bus (VB) driver. This VB driver provides a virtual device which manages the paths to all of
the devices that support failover. The lower half is a multi-path function driver for the devices using
the hardware ID created by the lower level filter driver. Windows calls these hardware paths
physical device objects (PDOs). The intermediate class driver creates two different type of devices
in the Windows device manager. The devices associated with the physical paths is shown as a
38 Installing and using Windows advanced path failover drivers