Note:  R/W stands for Read and Write operations. GLYPH<SM590000> I/O Rate R/W : The term  I/O  is used to describe any program, operation, or device that  transfers data to or from a computer, and to or from a peripheral device. Every transfer is  an output from one device and an input into another. Typically measured in IOPS. GLYPH<SM590000> Data Rate R/W : The data transfer rate (DTR) is the amount of digital data that is moved  from one place to another in a specific time. In case of Disk or Storage Subsystem, this  metric is the amount of data moved from a host to a specific storage device. Typically  measured in MBps. GLYPH<SM590000> Response time R/W : This is the time taken for a circuit or measuring device, when  subjected to a change in input signal, to change its state by a specified fraction of its total  response to that change. In case of Disk or Storage Subsystem, this is the time used to  complete an I/O operation. Typically measured in ms. GLYPH<SM590000> Cache Hit R/W : This is the percentage of times where read data or write data can be  found in cache or can find cache free space that it can be written to. GLYPH<SM590000> Average Data Block Size R/W : The block size is the unit of work for the file system. Every  read and write is done in full multiples of the block size. The block size is also the smallest  size on disk that a file can have. GLYPH<SM590000> Port-to-Local Node Queue Time (Send) : The average time in milliseconds that a send  operation spends in the queue before the operation is processed. This value represents  the queue time for send operations that are issued to other nodes that are in the local  cluster. A good scenario has less than 1 ms on average. GLYPH<SM590000> Port Protocol Errors (Zero Buffer Credit Percentage) : The amount of time, as a  percentage, that the port was not able to send frames between ports because of  insufficient buffer-to-buffer credit. The amount of time value is measured from the last time  that the node was reset. In Fibre Channel technology, buffer-to-buffer credit is used to  control the flow of frames between ports. In our experience less is better than more.  However, in a production environment this metric can be from 5% on average up to 20%  peak without affecting performance. GLYPH<SM590000> Port data rate (send and receive) : The average number of data in MBps for operations in  which the port receives or sends data. GLYPH<SM590000> Port Protocol Errors (Zero Buffer Credit Timer) : The number of microseconds that the  port is not able to send frames between ports because there is insufficient buffer-to-buffer  credit. In Fibre Channel technology, buffer-to-buffer credit is used to control the flow of  frames between ports. Buffer-to-buffer credit is measured from the last time that the node  was reset. This value is related to the data collection sample interval.  GLYPH<SM590000> Port Congestion Index:  The estimated degree to which frame transmission was delayed  due to a lack of buffer credits. This value is generally 0 - 100. The value 0 means there  was no congestion. The value can exceed 100 if the buffer credit exhaustion persisted for  an extended amount of time. When you troubleshoot a SAN, use this metric to help  identify port conditions that might slow the performance of the resources to which those  ports are connected. GLYPH<SM590000> Global Mirror (Overlapping Write Percentage) : The percentage of overlapping write  operations that are issued by the Global Mirror primary site. Some overlapping writes are  processed in parallel, and so they are excluded from this value.  GLYPH<SM590000> Global Mirror (Write I/O Rate) : The average number of write operations per second that  are issued to the Global Mirror secondary site. Keep in mind that IBM SAN Volume  Controller / Storwize systems have limited number of GM I/Os that can be delivered.  Chapter 8. Monitoring  387