Overview: The Next Generation Mass Storage Stack (September 2009)

DSF Details
There are two types of DSFs for mass storage: legacy DSFs and persistent DSFs. Both can be used to access a
given mass storage device independently, and both can coexist on a system.
Legacy DSF The only type of mass storage DSF available in releases prior to HP-UX 11i v3, so it is
associated with the legacy view. It is locked to a particular lunpath, and does not support
agile addressing. Each lunpath requires a different DSF, so a multi-pathed LUN has multiple
DSFs, one for each lunpath. (Note that on HP-UX 11i v3, legacy DSFs support multi-pathing
by default—that is, I/O requests to one legacy DSF may use any lunpath to the device. For
more information, see Disabling Multi-Pathing on Legacy DSFs
.)
A legacy DSF contains hardware path information such as SCSI controller, target, and LUN
in the device file name and minor number. The minor number field widths for controller
address (8 bits), target address (4 bits), and LUN address (3 bits) limit the system to 255
distinct controllers and 32768 distinct lunpaths. Systems with mass storage devices beyond
those limits are unable to address them using legacy DSFs.
The name and minor number also include any driver-specific options. For tape devices, this
includes tape density and rewind behavior.
The naming convention for legacy DSFs is described in mksf(1M) as follows:
/dev/dsk/cXtYdZ
/dev/rdsk/cXtYdZ
/dev/dsk/cXtYdZsP
/dev/rdsk/cXtYdZsP
/dev/rmt/cXtYdZ_options
/dev/rac/cXtYdZ_options
/dev/rscsi/cXtYdZ
Where
X is the instance number of the HBA
Y is the target address
Z is the LUN unit number
P is the optional partition number
Persistent DSF Associated with a LUN hardware path and seen in the agile view. Because it is based on
the LUN hardware path rather than the lunpath, a persistent DSF transparently supports
agile addressing. In other words, a persistent DSF is unchanged if the LUN is moved from
one HBA to another, moved from one switch or hub port to another, presented using a
different target port to the host, or configured with multiple hardware paths. Like the LUN
hardware path, the binding of DSF to a device persists across reboots, but is not
guaranteed to persist across installations.
The persistent DSF minor number contains no hardware path information, and its name
follows a simplified naming convention: /dev/subdir/classinstance
Where
subdir is the subdirectory for the device class, such as disk, tape, rdisk, or rtape
class is the device class, either disk or tape
instance is the instance number assigned to the device
Each class of device has its own set of instance numbers, so each combination of class and
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