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When oversubscription mode is enabled with redundant power (N+1 or N+N
redundancy), the chassis' total available power can be stretched beyond the
label rating (up to 120%). However, the power supplies are designed to sustain
this oversubscription for a limited time (approximately 1 second).
In healthy condition (all power supplies are in normal-operational mode), the
redundant power supplies provide the extra 20% power oversubscription load for
the rest of the normal-operational power supplies (none of the power supplies
are oversubscribed).
When redundant power supplies fail (that is, one power supply failure in N+1
mode, or up to N power supplies fail in N+N mode), the remaining
normal-operational power supplies provide the extra 20% power
oversubscription load. This extra power is provided only for a limited time to allow
the compute nodes to throttle to the lowest P-state to reduce their power usage
back to a supported range. By design, the compute nodes perform this action
quickly enough and operation continues.
Non-redundant mode: It is not possible to enable the oversubscription mode
without any power redundancy.
The Table 3-13 shows the consequences of redundancy failure in the chassis
with and without oversubscription mode.
Table 3-13 Consequences of power supply failure, depending on the oversubscription
Redundancy
Oversubscription
mode
mode
Non-redundant
Not available
Disabled
N+1
Enabled
Disabled
N+N
Enabled
a. Considering one power supply failure in non-redundant and N+1 mode, and
three power supplies failures in N+N mode.
b. Compute nodes are throttled only if they require more power than what is
available on the remaining power supplies.
c. The chassis is powered off only if after throttling the compute nodes, the
enclosure power requirement still exceeds the power that is available on the
remaining power supplies.
Consequences of redundancy failure
Compute nodes
b
might be throttled
Yes
No
Yes
No
Yes
Chapter 3. IBM NeXtScale n1200 Enclosure
a
Chassis might
power off
c
Yes
No
No
No
No
55
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