content_image/1003008.jpg
Now you can switch the role of the copies online (step 3), and test the performance of the new pool. If you are done testing your lower performance pool, you can split or delete the old copy in tier 1, or switch back to tier 1 in seconds, in case tier 2 pool did not meet your performance requirements. 

## 6.10 I/O throttling 

You can set a limit on the number of I/O operations that are accepted for a volume. The limit is set in terms of I/O operations per second (IOPS) or MBps. By default, no I/O throttling rate is set when a volume is created.I/O throttling is also referred to as I/O governing. 

Base the choice between I/O and MB as the I/O governing throttle on the disk access profile of the application. Database applications generally issue large amounts of I/O, but they transfer only a relatively small amount of data. In this case, setting an I/O governing throttle that is based on MBps does not achieve much. It is better to use an IOPS as a second throttle. 

At the other extreme, a streaming video application generally issues a small amount of I/O, but it transfers large amounts of data. In contrast to the database example, setting an I/O governing throttle that is based on IOPS does not achieve much, so it is better to use an MBps throttle. 

An I/O governing rate of 0  does not mean that zero IOPS (or MBps) can be achieved. It means that no throttle is set. 

Note:  Consider the following points: 

GLYPH<SM590000> I/O governing does not affect FlashCopy and data migration I/O rates. 

GLYPH<SM590000> I/O governing on a Metro Mirror and Global Mirror secondary volume does not affect the rate of data copy from the primary volume. 

## 6.10.1 Defining throttle on a volume 

To define a throttle on a volume, complete the following steps: 

1. From the Volumes window, select the wanted volume to throttle, as shown in Figure 6-54. 

<img src='content_image/1003008.jpg'>

