54 IBM NeXtScale System Planning and Implementation Guide N+N ModeHalf of the power supplies that are installed are used as redundant. Thefailure of up to half the number of the power supplies is allowed withoutaffecting the system’s operation or performance (performance can beaffected if oversubscription mode is enabled). This mode is useful if you havetwo power sources from two separate PDU for example.This mode can be enabled with oversubscription mode.3.8.4 Power supply oversubscriptionBy using oversubscription, users can make the most of the extra power from theredundant power supplies when the power supplies are in healthy condition.For example, when oversubscription is enabled with N+1 redundancy mode, thetotal power that is available is equivalent to No Redundancy mode with six powersupplies in the chassis. This means we can count on the power of six powersupplies instead of five for normal operation.When oversubscription mode is enabled with redundant power (N+1 or N+Nredundancy), the total available power is 120% of the label ratings of the powersupplies. So, for a 900 W power supply, it can be oversubscribed to 900 W x120% = 1080 W.For example, this means that with an N+1 power policy and six power supplies,instead of 5 x 900 W (4500 W) of power, you have 5 x 900 W x 120% (5400 W) ofpower that is available to the compute nodes.Table 3-12 shows the power budget that is available, depending on theredundancy and oversubscription mode that is selected.Table 3-12 Power budget for 6 x 900 W power suppliesRedundancy Mode Oversubscription mode Power budgetaa. The power budget that is presented in this table is based on power supplyratings. Actual power budget can vary.Non-redundant Not available 5400 W (= 6 x 900 W)N+1Disabled 4500 W (= 5 x 900 W)Enabled 5400 W (= 5 x 900 W x 120%)N+NDisabled 2700 W (= 3 x 900 W)Enabled 3240 W (= 3 x 900 x 120%)
