Use the Performance Monitor data to make storage subsystem tuning decisions, as described in the table below.
Data
Implications for Performance Tuning
Total I/Os
This data is useful for monitoring the I/O activity of a specific controller and a specific logical drive, which can help identify possible high-traffic I/O areas.
If I/O rate is slow on a logical drive, try increasing the array size using Array >> Add Free Capacity (Drives). Or you might try a better matching I/O size to segment size.
You might notice a disparity in the Total I/Os (workload) of controllers, for example, the workload of one controller is heavy or is increasing over time while that of the other controller is lighter or more stable. In this case, consider changing the controller ownership of one or more logical drives to the controller with the lighter workload. Use the logical drive Total I/O statistics to determine which logical drives to move.
If you notice the workload across the storage subsystem (Storage Subsystem Totals Total I/O statistic) continues to increase over time while application performance decreases, this might indicate the need to add additional storage subsystems 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 logical drive to determine actual application behavior. If there is a low percentage of read activity relative to write activity, consider changing the RAID level of an array from RAID 5 to RAID 1 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 logical drives may be low or trending downward. This may indicate inherent randomness in access patterns, or, at the storage subsystem or controller level, this can indicate the need to install more controller cache memory if you do not have the maximum amount of memory installed.
If an individual logical drive is experiencing a low cache hit percentage, consider enabling cache read ahead for that logical drive. Cache read aheadcan increase the cache hit percentage for a sequential I/O workload.
Determining the effectiveness of a logical drive cache read ahead multiplier
To determine if your I/O has sequential characteristics, try enabling a conservative cache read-ahead multiplier (4, for example) using the Logical Drive >> Properties option. Then, examine the logical drive cache hit percentage to see if it has improved. If it has, indicating that your I/O has a sequential pattern, enable a more aggressive cache read-ahead multiplier (8, for example). Continue to customize logical drive cache read-ahead to arrive at the optimal multiplier (in the case of a random I/O pattern, the optimal multiplier is zero).
Current KB/sec and Maximum KB/sec
The transfer rates of the controller 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 subsystem.
Consider a storage subsystem, equipped with Fibre Channel controllers, that supports a maximum transfer rate of 100 MB/second, (100,000 KB per second). Your storage subsystem typically achieves an average transfer rate of 20,000 KB/second. (The typical I/O size for your applications is 4K, with 5,000 I/Os transferred per second for an average rate of 20,000 KB/second.) 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 per second. However, if your typical I/O size is large, a transfer rate within a range of 80,000 - 90,000 KB per second might be achieved.
Wide Ultra SCSI supports sustained data transfer rates of up to 40 MB/second with large host I/O sizes. The maximum transfer rates with relatively small I/O sizes, 4096 bytes, is approximately 17 MB/second.
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, segment size, and number of drives in the arrays or storage subsystem. 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 logical drive. Experiment to determine the optimal segment size, or use the file system or database block size.
Higher write I/O rates are experienced with write caching enabled compared to disabled. In deciding whether to enable write caching for an individual logical drive, 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.