Dell PowerFlex 4.6.x Technical Overview

Rebuild throttling

The rebuild throttling policy determines the priority of rebuild I/Os versus application I/Os when accessing SDS devices. The possible rebuild throttling policies are no limit on rebuild I/Os, limit concurrent I/Os per SDS device, favor application I/Os and dynamic bandwidth throttling.

Rebuild throttling sets the rebuild priority policy for a storage pool. The policy determines the priority between the rebuild I/O and the application I/O when accessing SDS devices. Note that application I/Os are continuously served.

Applying rebuild throttling will increase the time the system is exposed with a single copy of some of the data, but it will reduce the impact on the application. You must attempt to choose the right balance between the two.

The following priority policies may be applied:

• No limit: No limit on rebuilding I/Os. Any rebuild I/O is submitted to the device immediately, without further queuing.
  NOTE:Rebuild I/Os are relatively large and hence setting this policy will speed up the rebuild, but will have the maximal effect on the application I/O.
• Limit concurrent I/O: Limit the number of concurrent rebuild I/Os per SDS device (default). The rebuild I/Os are limited to a predefined number of concurrent I/Os. When the limit is reached, the next incoming rebuild I/O waits until the completion of a currently performed rebuild I/O. This will complete the rebuild quickly for best reliability, however, there is a risk of host application impact.
• Favor application I/O: Limit rebuild in both bandwidth and concurrent I/Os. The rebuild I/Os are limited both in bandwidth and in the amount of concurrent I/Os. As long as the number of concurrent rebuild I/Os, and the bandwidth they consume, do not exceed the predefined limits, rebuild I/Os will be served. Once either threshold is reached, the rebuild I/Os wait until both I/O and bandwidth are below their thresholds. For example, setting the value to 1 guarantees the device has one concurrent rebuild I/O at any given moment, which ensures the application I/Os only wait for 1 rebuild I/O at worst case. This imposes bandwidth on top of the limit concurrent I/Os option, which is a prerequisite to using this policy.
• Dynamic bandwidth throttling: This policy is similar to favor application I/O, but extends the interval in which application I/Os are considered to be flowing by defining a minimal quiet period. This quiet period is defined as a certain interval in which no application I/Os occurred. Note that the limits on the rebuild bandwidth and concurrent I/Os are still imposed.
• Default values:
  • The default policy for rebuild is: Limit Concurrent I/O
  • Rebuild concurrent I/O Limit: 1 concurrent I/O
    NOTE: Rebuild throttling affects system performance and should only be used by advanced users.