Learn About Performance Monitor Data

Data

Implications for Performance Tuning

Total I/Os

This data is useful for monitoring the I/O activity of a specific  and a specific , which can help identify possible high-traffic I/O areas.

If I/O rate is slow on a virtual disk, try increasing the  size by selecting Disk Group >> Add Free Capacity (Physical Disks).

You might notice a disconsistency in the Total I/Os (workload) of RAID controller modules, for example, the workload of one RAID controller module is heavy or is increasing over time while that of the other RAID controller module is lighter or more stable. In this case, consider changing the  of one or more virtual disks to the RAID controller module with the lighter workload. Use the virtual disk Total I/O statistics to determine which virtual disks to move.

If you notice that the workload across the storage array (in the Performance Monitor window, see the Total I/Os column of the Storage Array Totals row) continues to increase over time while application performance decreases, this might indicate the need to add additional storage arrays to your enterprise so that you can continue to meet application needs at an acceptable performance level.

Read Percentage

Use the Read Percentage for a virtual disk to determine actual application behavior. If there is a low percentage of read activity relative to write activity, consider changing the  level of a disk group from  to  for faster performance.

Cache Hit Percentage

A higher percentage is desirable for optimal application performance. There is a positive correlation between the cache hit percentage and 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, or, at the storage array or RAID controller module level, this trend might indicate the need to install more RAID controller module  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  can increase the cache hit percentage for a sequential I/O workload.

Current KB/sec and Maximum KB/sec

The transfer rates of the RAID controller module are determined by the application I/O size and the I/O rate. Generally, small application I/O requests result in a lower transfer rate but provide a faster I/O rate and shorter response time. With larger application I/O requests, higher throughput rates are possible. Understanding your typical application I/O patterns can help you determine the maximum I/O transfer rates for a given storage array.

Consider a storage array, equipped with  RAID controller modules, that supports a maximum transfer rate of 100 MB/second (100,000 KB/s). Your storage array typically achieves an average transfer rate of 20,000 KB/s. (The typical I/O size for your applications is 4 K, with 5,000 I/Os transferred per second for an average rate of 20,000 KB/s.) In this case, I/O size is small. Because there is system overhead associated with each I/O, the transfer rates will not approach 100,000 KB/s. However, if your typical I/O size is large, a transfer rate within a range of 80,000 to 90,000 KB/s might be achieved.

Wide Ultra SCSI supports sustained data transfer rates of up to 40 MB/s with large  I/O sizes. The maximum transfer rates with relatively small I/O sizes, 4096 bytes, is approximately 17 MB/s.

Current I/O per sec and Maximum I/O per sec

Factors that affect I/Os per second include access pattern (random or sequential), I/O size, RAID level, , and number of physical disks in the disk groups or storage array. The higher the cache hit rate, the higher I/O rates will be.

Performance improvements caused by changing the segment size can be seen in the I/Os per second statistics for a virtual disk. Experiment to determine the optimal segment size, or use the file system or database block size.

Higher write I/O rates are experienced with  enabled compared to disabled. In deciding whether to enable write caching for an individual virtual disk, consider the current and maximum I/Os per second. You should expect to see higher rates for sequential I/O patterns than for random I/O patterns. Regardless of your I/O pattern, it is recommended that write caching be enabled to maximize I/O rate and shorten application response time.