Administrator Guide

Table 4. Performance data implications (continued)
Performance Data Implications for Performance Tuning
database block size. For more information about segment size and
performance, see the related topics listed at the end of this topic.
The higher the cache hit rate, the higher I/O rates will be. Higher
write I/O rates are experienced with write caching enabled
compared to disabled. In deciding whether to enable write caching
for an individual virtual disk, look at the current IOPS and the
maximum IOPS. You should see higher rates for sequential I/O
patterns than for random I/O patterns. Regardless of your I/O
pattern, enable write caching to maximize the I/O rate and to
shorten the application response time. For more information about
read/write caching and performance, see the related topics listed
at the end of this topic.
MBs/sec See IOs/sec.
I/O Latency, ms Latency is useful for monitoring the I/O activity of a specific
physical disk and a specific virtual disk and can help you identify
physical disks that are bottlenecks.
Physical disk type and speed influence latency. With random I/O,
faster spinning physical disks spend less time moving to and from
different locations on the disk.
Too few physical disks result in more queued commands and a
greater period of time for the physical disk to process the
command, increasing the general latency of the system.
Larger I/Os have greater latency due to the additional time
involved with transferring data.
Higher latency might indicate that the I/O pattern is random in
nature. Physical disks with random I/O will have greater latency
than those with sequential streams.
If a disk group is shared among several virtual disks, the individual
virtual disks might need their own disk groups to improve the
sequential performance of the physical disks and decrease latency.
If inconsistency exists with physical disks of a common disk group.
This condition might indicate a slow physical disk.
With disk pools, larger latencies are introduced and uneven
workloads might exist between physical disks making the latency
values less meaningful and in general higher.
Cache Hit Percentage
A higher cache hit percentage is desirable for optimal application
performance. A positive correlation exists between the cache hit
percentage and the I/O rates.
The cache hit percentage of all of the virtual disks might be low or
trending downward. This trend might indicate inherent randomness
in access patterns. In addition, at the storage array level or the
RAID controller module level, this trend might indicate the need to
install more RAID controller module cache memory if you do not
have the maximum amount of memory installed.
If an individual virtual disk is experiencing a low cache hit
percentage, consider enabling dynamic cache read prefetch for
that virtual disk. Dynamic cache read prefetch can increase the
cache hit percentage for a sequential I/O workload.
28 About your MD Series storage array