Read I/O capability formula

The read I/O capability formula is used by IBM Spectrum Control to estimate the workload capacity of managed disk (MDisk) groups.

The estimated workload capacity is compared with the actual workload capacity to assess the most active and least active MDisk groups in a storage subsystem. To monitor the workload activity of MDisk groups, you can run customized reports such as the MDisk Groups - Workload Activity Report. This formula is used for MDisk groups in the following storage systems:
  • SAN Volume Controller
  • Storwize V3500
  • Storwize V3700
  • Storwize V7000
  • Storwize V7000 Unified

The formula that is used by IBM Spectrum Control to estimate the read I/O capability rate is:

[Estimated Read I/O Workload * Type of Disk I/O Capability * Number of Disks] / [Estimated Read I/O Workload * (1 − Cache Hit Ratio) + Estimated Write I/O Workload * RAID Type Weighted I/O Ratio]

There are two types of value that are used by the formula:
  • Fixed values provided by IBM Spectrum Control
  • Variable values that you must specify for each MDisk group

The following fixed values are used by the formula:

Estimated Read I/O Workload
The estimated read I/O workload of an MDisk group. The estimate is set to a fixed value of 0.7 (70%).
Estimated Write I/O Workload
The estimated write I/O workload of an MDisk group. The estimate is set to a fixed value of 0.3 (30%).

You specify the following variable values for each MDisk group:

Type of Disk I/O Capability
When you specify the type of disk that the back-end pool uses:
  • You can accept the default I/O capability per second of the disk
  • You can enter a new value for the I/O capability per second of the disk
For example, the default value for the I/O capability rate per second of fiber types of disk with 15,000 revolutions per minute (rpm) is 150 I/O operations per second. To change the I/O capability for a type of disk, you issue the setbackenddisktype command and you enter a new value for the -iops parameter.
Number of Disks
You specify the number of physical disks that the back-end pool uses.
Cache Hit Ratio
When you specify the type of storage system that the back-end pool uses:
  • You can accept the default cache hit ratio for the storage system
  • You can enter a new value for the cache hit ratio of the storage system
For example, the default cache hit ratio of the XIV is 0.7 (70%). To change the cache hit ratio for a type of storage system, you issue the setbackendtype command and you enter a new value for the -cachehit parameter.
RAID Type Weighted I/O Ratio
When you specify the type of RAID that the back-end pool uses:
  • You can accept the default value for the weighted I/O ratio of the type of RAID
  • You can enter a new value for the weighted I/O ratio of the type of RAID
For example the weighted I/O ratio for RAID5 types of RAID is 4:1. To change the weighted I/O ratio for a type of RAID, you issue the setbackendraidtype command and you enter a new value for the -weightedio parameter.
Tip: In the IBM Spectrum Control GUI, you can set the values for the back-end properties of an MDisk group. You can also use the IBM Spectrum Control CLI called tpctool to set the values for back-end MDisk groups. See the setarray command. To change the variable values that are used by the Read I/O capability formula, you must use the IBM Spectrum Control CLI.

Example

The following values have been set for a back-end pool. The type of storage system is the XIV. The type of disk is F15 (Fiber - 15 000 rpm), and the type of RAID is X.
Name of variable Value
Cache Hit Ratio 0.5 (50%)
Type of Disk I/O Capability 150
RAID Type Weighted I/O Ratio 4
Number of Disks 10

Based on the values that are provided, the read I/O capability per second is:

[0.7 * 150 * 10] / [0.7 * (1 − 0.5) + 0.3 * 4]

The read I/O capability per second rate is 677. (The result of the calculation is rounded to the nearest whole number.)