The parameters passed in the Volume Characteristics Structure (VCS) are
used by FSDs to optimize disk access and placement of file system structures
on an advisory basis. All values reflect the physical parameters of the
logical volume, as if it were a single physical device (that is, whether
the media is partitioned or not). This data structure is passed by reference
and is maintained and updated by the physical device driver, as necessary.
It is expected that the physical device driver would maintain a separate
VCS for each logical volume supported. A VCS has the following format:
┌──────────────────────────────┬──────────────────────────────┐
│Field │Length │
├──────────────────────────────┼──────────────────────────────┤
│VolDescriptor │WORD │
├──────────────────────────────┼──────────────────────────────┤
│AvgSeekTime │WORD │
├──────────────────────────────┼──────────────────────────────┤
│AvgLatency │WORD │
├──────────────────────────────┼──────────────────────────────┤
│TrackMinBlocks │WORD │
├──────────────────────────────┼──────────────────────────────┤
│TrackMaxBlocks │WORD │
├──────────────────────────────┼──────────────────────────────┤
│Head Per Cylinder │WORD │
├──────────────────────────────┼──────────────────────────────┤
│VolCylinderCount │DWORD │
├──────────────────────────────┼──────────────────────────────┤
│VolMedianBlock │DWORD │
├──────────────────────────────┼──────────────────────────────┤
│MaxSGList │WORD │
└──────────────────────────────┴──────────────────────────────┘
VolDescriptor
A bit field, defined as follows:
Bit 0
If set, volume resides on removable media
Bit 1
If set, volume is read-only
Bit 2
If set, average seek time independent
of position (RAM disk)
Bit 3
If set, outboard cache supported
Bit 4
If set, scatter/gather supported by adapter
Bit 5
If set, ReadPrefetch supported
Bits 6-15
AvgSeekTime
The average seek time (in milliseconds) in servicing
this volume. If the seek time is unknown, FFFFH is to be specified. Can
be 0 for RAM disks.
AvgLatency
The
average rotational latency (in milliseconds) for the device servicing this
volume. If the average latency is unknown, FFFFH is to be specified. Latency
can be 0 for RAM disks.
TrackMinBlocks
The
number of blocks available on the smallest capacity track; if unknown or
not applicable, a value of 1 is specified.
TrackMaxBlocks
The number of blocks available on the largest capacity
track; if unknown or not applicable, a value of 1 is specified.
Heads
Per Cylinder
The number of heads per cylinder; if
unknown or not applicable, a value of 1 is specified.
VolCylinderCount
The number of cylinders in the volume; if unknown
or not applicable, the number of allocation blocks (sectors) is used.
VolMedianBlock
The number of the block, which is in the center
of the volume with respect to seek time (that is, the block with the smallest
average seek time).
MaxSGList
The
maximum number of scatter and gather list entries, which can be directly
submitted to the adapter servicing this volume with one low-level I/O command.
File systems submitting extended commands with scatter and gather lists
greater than MaxSGList entries must ensure that the cumulative byte count
of each MaxSGList entry in the list is a multiple of the sector size. This
field is set to 0, if the volume is serviced by an adapter that does not
directly support scatter/gather lists. See Scatter/Gather
Descriptor for details on scatter/gather lists passed in extended requests.
[Back: Driver Capabilities Structure (DCS)]
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