Dell PowerFlex 4.5.x Technical Overview

Fault sets

A fault set defines a group of SDSs that are likely to go offline simultaneously. Defining a fault set configures PowerFlex to mirror the devices in the set.

A fault set is a logical entity that contains a group of SDSs within a protection domain that have a higher chance of going offline together such as a group of SDSs that are all powered in the same rack. By grouping them into a fault set, you are configuring PowerFlex to mirror the data for all devices in this fault set. The mirroring should take place on SDSs that are outside of this fault set.

When defining fault sets, the term fault units refers to either a fault set, or an SDS not associated with a fault set.

There must be enough capacity within at least three fault units to enable mirroring.

If fault sets are defined, you can use any combination of fault units, for example:

• SDS1, SDS2, SDS3
• FS1, SDS1, SDS2
• FS1, FS2, SDS1
• FS1, FS2, FS3

In the following figure there are three protection domains. The middle one (fully depicted) consists of seven SDSs, each with two storage devices.

To use fault sets, you must work in the following order:

1. Ensure that a protection domain exists, or add a new one.
2. Ensure that a storage pool and fault sets (minimum of three fault units) exist, or add new ones.
3. Add the SDS, designating the protection domain and fault set, and at the same time, adding the SDS devices into a storage pool.

   The automated deployment and installation tools follow this order automatically.

   You can only create and configure fault sets before adding SDSs to the system, and configuring them incorrectly may prevent the creation of volumes. An SDS can be added to a fault set only during the creation of the SDS.

You can also add fault sets when adding SDS nodes after initial installation.