54 IBM NeXtScale System Planning and Implementation Guide
N+N Mode
Half of the power supplies that are installed are used as redundant. The
failure of up to half the number of the power supplies is allowed without
affecting the system’s operation or performance (performance can be
affected if oversubscription mode is enabled). This mode is useful if you have
two power sources from two separate PDU for example.
This mode can be enabled with oversubscription mode.
3.8.4 Power supply oversubscription
By using oversubscription, users can make the most of the extra power from the
redundant power supplies when the power supplies are in healthy condition.
For example, when oversubscription is enabled with N+1 redundancy mode, the
total power that is available is equivalent to No Redundancy mode with six power
supplies in the chassis. This means we can count on the power of six power
supplies instead of five for normal operation.
When oversubscription mode is enabled with redundant power (N+1 or N+N
redundancy), the total available power is 120% of the label ratings of the power
supplies. So, for a 900 W power supply, it can be oversubscribed to 900 W x
120% = 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) of
power that is available to the compute nodes.
Table 3-12 shows the power budget that is available, depending on the
redundancy and oversubscription mode that is selected.
Table 3-12 Power budget for 6 x 900 W power supplies
Redundancy Mode Oversubscription mode Power budget
a
a. The power budget that is presented in this table is based on power supply
ratings. Actual power budget can vary.
Non-redundant Not available 5400 W (= 6 x 900 W)
N+1
Disabled 4500 W (= 5 x 900 W)
Enabled 5400 W (= 5 x 900 W x 120%)
N+N
Disabled 2700 W (= 3 x 900 W)
Enabled 3240 W (= 3 x 900 x 120%)
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