Databases Reference
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Another way to think about this is to consider a few examples.
In the first example in Figure 12-1, you will consider a single disk system where the disk has four
partitions, and each of these is mounted as a separate disk C, D, E, F. Initially you might think that the
physical disk counters would show just the one physical disk, but because the disk is partitioned before
it's presented to the OS, the physical disk counters will still show four disk instances. The logical disk
counters will also show four disk instances.
Physical Disk 0
Logical Disk 0
Logical Disk 1
Logical Disk 3
Logical Disk 2
Figure 12-1
In the second example in Figure 12-2, consider the other extreme of a small RAID array of four disks
configured into a simple stripe set. The array controller presents this to the OS as a single disk which is
mounted as D. You might think that the physical disk counters would show the four disks in the drive
array, but the OS doesn't know anything about four disks, it only knows about the D disk presented to
it by the Array controller card. The logical disk counters will again show just a single instance for the
Ddrive.
In the third example in Figure 12-3, look at the four disks attached to a four channel SATA card and
mounted as four separate disks. These disks are then combined into a software RAID set inside the
OS, and mounted as D. In this case the logical disk counters will show a single instance D whereas
the physical disk counters will show four instances, one for each of the disks.
Unfortunately there isn't a Best Practice that covers which set of counters to monitor. In some cases you
will want to use the Physical Disk counters. In other cases such as when using mount points, you will
want to monitor the Logical Disk counters. Having an understanding of where each set of counters is
monitoring will help you make the decision about which set is right for your case.
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