eG Monitoring
 

Measures reported by DiskActivityTest

When executed on Windows, Solaris, AIX, and HP-UX systems, this test reports statistics pertaining to the input/output utilization of each physical disk on a system. On Linux systems, the test will return the input/output utilization of each “device” on the system. The device name is in the format “hdiskn” for 2.2 kernels, where “n” is the device number. For newer Linux kernels though, the device name is displayed as “devm-n”, where m is the major number of the device, and n a distinctive number. The measures reported by this test are as follows:

Measurement Description Measurement Unit Interpretation
Percent_busy Percentage of elapsed time during which the disk is busy processing requests (i.e.,reads or writes) Percent Comparing the percentage of time that the different disks are busy, an administrator can determine whether the application load is properly balanced across the different disks. The detailed diagnosis of this measure (if enabled) lists the processes executing on the disk and the IO operations performed by them.
Percent_disk_reads Percentage of elapsed time that the selected disk drive is busy servicing read requests Percent  
Percent_disk_writes Percentage of elapsed time that the selected disk drive is busy servicing write requests Percent  
Disk_read_time Average time in seconds of a read of data from the disk Secs  
Disk_write_time Average time in seconds of a write of data from the disk Secs  
Avg_queue_length The average number of both read and write requests that were queued for the selected disk during the sample interval Number The detailed diagnosis of this measure (if enabled) lists the processes executing on the disk and the IO operations performed by them.
Current_queue_length The number of requests outstanding on the disk at the time the performance data is collected Number This measure includes requests in service at the time of the snapshot. This is an instantaneous length, not an average over the time interval. Multi-spindle disk devices can have multiple requests active at one time, but other concurrent requests are awaiting service. This counter might reflect a transitory high or low queue length, but if there is a sustained load on the disk drive, it is likely that this will be consistently high. Requests experience delays proportional to the length of this queue minus the number of spindles on the disks. This difference should average less than two for good performance.
Read_rate Indicates the number of reads happening on a logical disk per second. Reads/Sec A dramatic increase in this value may be indicative of an I/O bottleneck on the server.
Data_read_rate The rate at which bytes are transferred from the disk during read operations KBytes/Sec A very high value indicates an I/O bottleneck on the server.
Write_rate Indicates the number of writes happening on a local disk per second. Writes/Sec A dramatic increase in this value may be indicative of an I/O bottleneck on the server.
Data_write_rate The rate at which bytes are transferred from the disk during write operations KBytes/Sec A very high value indicates an I/O bottleneck on the server.
Service_time Indicates the average time that this disk took to service each transfer request ( i.e., the average I/O operation time). Secs A sudden rise in the value of this measure can be attributed to a large amount of information being input or output. A consistent increase however, could indicate an I/O processing bottleneck.
Queue_time Indicates the average time that transfer requests waited idly on queue for this disk. Secs Ideally, the value of this measure should be low.
IO_time Indicates the avarage time taken for read and write operations of this disk. Secs The value of this measure is the sum of the values of the Disk service time and Disk queue time measures.

A consistent increase in the value of this measure could indicate a latency in I/O processing.
Avg_io_read_size Indicates the average number of bytes transferred from disk during read operations. KB Larger I/Os tend to have higher latency (for example, BACKUP/RESTORE operations issue 1 MB transfers by default).

These measures are reported for Windows VMs only.
Avg_io_write_size Indicates the average number of bytes transferred into disk during write operations. KB
Split_io Reports the rate at which the operating system divides I/O requests to the disk into multiple requests. Splits/Sec A split I/O request might occur if the program requests data in a size that is too large to fit into a single request or if the disk is fragmented. Factors that influence the size of an I/O request can include application design, the file system, or drivers. A high rate of split I/O might not, in itself, represent a problem. However, on single-disk systems, a high rate for this counter tends to indicate disk fragmentation.

This measure is reported for Windows VMs only.

Note 1:

  • For this test to report measures on Unix systems, the sysstat package must be installed on the server (check for the existence of the iostat command on the target system).
  • The following measures are not available for Linux systems: Percent_disk_reads, Percent_disk_writes, Avg_queue_length, Current_queue_length. If the sysstat version installed on the target server is less than 4.0.7, the following measures also will not be available - Data_read_rate, Data_write_rate, Percent_busy.
  • The following measures are not available on Solaris - Percent_disk_reads, Percent_disk_writes, Current_queue_length. Furthermore, the eG agent can only measure the time for access to the disk (not differentiating between read and write times). This test reports the Disk_read_time and the Disk_write_time to be the disk access time reported by the operating system.
  • The following measures are not available for HPUX environments - Percent_disk_reads, Percent_disk_writes, Current_queue_length. As the operating system only provides the overall transfer rate to and from the disk, the eG agent reports this value for both the Read_rate and the Write_rate. Likewise, the Data_read_rate and Data_write_rate are also reported as the same values (i.e., equal to the data transfer rate from the disk). Likewise, the Disk_read_time and Disk_write_time are also reported as the same values (i.e., equal to the seconds per average seek).
  • The following measures are not available for AIX environments - Percent_disk_reads, Percent_disk_writes, Disk_read_time, Disk_write_time, Avg_queue_length, and Current_queue_length. As the operating system only provides the overall transfer rate to and from the disk, the eG agent reports this value for both the Read_rate and the Write_rate.
  • Detailed diagnosis will not be available for systems operating on AIX platforms.