Dell PowerVault MD3820f - Manual (Page 37)

Performance Data Implications for Performance Tuning
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 a disparity 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
37