## Management switch failure 

This failure is tricky because the system remains online despite the host being unable to communicate with various components, such as RPCs and AMMs. Therefore, the system cannot power cycle SPUs, perform a host failover, or receive confirmations that SPUs completed starting. If the database pauses or stops, it cannot be brought back up until the failed management switch is replaced. 

It is critical to replace the switch as soon as possible. Failures of this component are rare, but must be treated with critical attention when they occur. 

The replacement requires a four-hour system outage. 

## Host disk failure 

The hosts use RAID 10 and include a spare disk. Depending on the model, seven disks are used (three mirror pairs of two and a spare) or five disks (two mirror pairs and a spare). Failure of a single disk is not cause for alarm because the spare disk takes over and RAID 10 is preserved with data mirroring. 

Multiple host disk failures can be serious problem. If the system loses both copies of the data, the host fails and must be rebuilt. The rebuilding process requires a 10-hour system outage. Replacing a host disk is an online process. 

## Host failure 

A total host failure can be the result of many issues, such as a motherboard or RAID controller failure. 

Whatever the cause, your system is affected only if the issue occurs on the active host. Because the standby host is on standby and database connections are not running through the host, failure does not affect query processing. 

If the active host fails, the database is moved from the failed host to the standby host so that processing can resume. However, this move interrupts all processing and database connections. After 15 minutes, the database often is back online and running on the second host, ready to resume running queries. 

The outage that is needed to fix a host depends on the components that must be replaced. There is no performance effect on the environment; however, you cannot have a high availability system until the host is repaired. If the last remaining host is lost, all of the data that is associated with that host also is lost. 

## Rack power supply failure 

When we describe a rack power supply failure, we are referring to each rack. Each rack features its own independent power from other racks. The one exception is N3001/N200x-002 models, whose racks are referred to as a quarter rack. These racks require less power and are powered by two long RPCs that are directly connected to the datacenter's power. One RPC represents the rack's left side power and the other the right-side power. 

The following types of power supplies provide power to the entire rack: 

From your datacenter, there are two power cables that are connected to the rack. These cables are  from different power domains to sustain redundancy and are connected to the PDUs. 

The PDUs are unmanageable and the system does not monitor them. Although it is rare for a PDU to fail, a failure is easy to identify because an entire side of a rack does not have power. 

The second type of power supply is the RPCs, which are monitored. By logging in to the RPCs, any  socket on that power supply can be and powered down or up. 

The power layout features two power cables from the datacenter that are connected to two PDUs. Each PDU supplies power to two RPCs and the BladeCenters. 

