Physical Objects—Physical
Disks |
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A physical disk is the basic storage
device (media) where the data is ultimately stored. You can access
the data on a physical disk by using a device name to locate the disk.
The physical disk device name varies with the computer system you
use. Not all parameters are used on all systems. Typical device
names are of the form c#t#d#, where:
c# specifies the controller
t# specifies the target ID
The figure, Figure 1-1 “Physical
Disk Example” shows how a physical disk and device name (devname)
are illustrated in this document. For example, device name c0t0d0 is
the entire hard disk connected to controller number 0 in
the system, with a target ID of 0, and
physical disk number 0.
VxVM writes identification information on physical disks
under VxVM control (VM disks). VxVM disks can be identified
even after physical disk disconnection or system outages. VxVM can
then re-form disk groups and logical objects to provide failure
detection and to speed system recovery.
For HP-UX 11.x, all the disks are treated and accessed by VxVM as entire
physical disks using a device name such as c#t#d#.
Performing I/O to disks is a relatively slow
process because disks are physical devices that require time to
move the heads to the correct position on the disk before reading
or writing. If all of the read or write operations are done to individual
disks, one at a time, the read-write time can become unmanageable.
Performing these operations on multiple disks can help to reduce
this problem.
A disk array is a collection of physical
disks that VxVM can represent to the operating system as one
or more virtual disks or volumes. The volumes created by VxVM look
and act to the operating system like physical disks. Applications
that interact with volumes should work in the same way as with physical
disks.
Figure 1-2 “How VxVM Presents
the Disks in a Disk Array as Volumes to the Operating System” illustrates how VxVM represents
the disks in a disk array as several volumes to the operating system.
Data can be spread across several disks within an array to
distribute or balance I/O operations across the disks. Using parallel
I/O across multiple disks in this way improves I/O performance by
increasing data transfer speed and overall throughput for the array.
Some disk arrays provide
multiple ports to access their disk devices. These ports, coupled
with the host bus adaptor (HBA) controller and any data bus or I/O
processor local to the array, make up multiple hardware paths to
access the disk devices. Such disk arrays are called multipathed disk
arrays. This type of disk array can be connected to host
systems in many different configurations, (such as multiple ports
connected to different controllers on a single host, chaining of
the ports through a single controller on a host, or ports connected
to different hosts simultaneously). For more detailed information,
see Chapter 3 “Administering
Dynamic Multipathing (DMP)”.
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