Chapter 3. IBM NeXtScale n1200 Enclosure 55
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.
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
Non-redundant mode: It is not possible to enable the oversubscription mode
without any power redundancy.
Redundancy
mode
Oversubscription
mode
Consequences of redundancy failure
a
a. Considering one power supply failure in non-redundant and N+1 mode, and
three power supplies failures in N+N mode.
Compute nodes
might be throttled
b
b. Compute nodes are throttled only if they require more power than what is
available on the remaining power supplies.
Chassis might
power off
Non-redundant Not available Yes Yes
c
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.
N+1
Disabled
No No
Enabled Yes
No
N+N
Disabled
No No
Enabled Yes
No
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