ML350 Gen9 x4 RPS with 4 PSUs behaves strange

[chrgarling]

Hi there,

we have two ML350 Gen9 with x4 RPS extension and four 800W Platinum PSUs (each server). We expected that two PSUs (double redundancy) are enough to run this machine, but it seems like three PSUs are the minimum requirement. This situation does not fit with our UPS / power supply setup without buying additional hardware. I have tested the setup with the BIOS settings "balanced", "high eff. (even backup)" and "high eff. (auto)" with no luck. Always three plugged in PSUs are OK, but with two plugged in PSUs the system shuts down and goes into failure mode (red blinking LED), a power cycle is needed to get it on back again.

Is this behavior the right one? Maybe we missed a setting? Any chance to get it working with two PSUs or are three PSUs the correct minimum requirement?

Each machine is equipped with:

2 cpus (4 cores)

4x 32GB RAM

2x 600GB SAS (system disks)

10x 1,2TB SAS (data disks)

1x P441 Controller

3x P840 Controller

3x SN1000Q 2 port HBA

Any tips, ideas and also questions are welcome!

Regards, Christian Garling

[effihan1]

Question, how did you solve this problem?

Regards

Hans

[Nestorov]

Hi, chrgarling,

it seems this is a rather common misconception, that the x4 rps should somehow offer a different redundancy mode from the x2 rps backplane. The server actually behaves quite normal, because both the x2 rps and the x4 rps backplane offer a redundancy of only 1 PSU at most and as the x2 rps backplane requires a minimum of one PSU, the x4 rps backplane requires a minimum of 4 PSUs and needs all 4 installed so that the server can have a redundancy in the power subsystem.

The idea behind the conception of the x4 rps backplane was that the server offers a lot of expansion options with its grand total of  up to 9 PCIe cards, and up to 6 HDD backplanes (or internal tape drive + 5 HDD backplanes for instance) which might consume a lot of power and actually require more than the output of even the 1400W Flex Slot Platinum Plus Hot Plug Power Supply (e.g. if you have a couple of NVidia graphic cards -- say the K6000 (~tdp of 225W), 6xHDD backplanes that would require at least two Smart Array Controllers + 2 SAS expander cards and say all 6 Backplanes are filled with HDDs for a total of 48 HDDs plus say an additioanl NIC or two, 2 of the higher end CPUs with a TDP of 145W each + 12 dimms per CPU i think you can get the picture).

So regardless of the PSU backplane you choose, you need to have n-1 PSUs installed for the server to run and all slots populated if you want a redundancy of one PSU.

You can use the HPE Power Advisor online edition to calculate exactly how much the components you'd want to install in your ML350 gen9 will consume:

https://paonline56.itcs.hpe.com/?Page=Index

but a word of advise would be to always overprovision and then decide whether you need the x4 rps backplane at all or if the x2 rps backplane would be more than sufficient.

Additional links:

* Quickspecs for the ML350 gen9:

http://www8.hp.com/h20195/v2/getpdf.aspx/c04375628.pdf

--> p. 28 gives a bit of info about the x4 rps Backplane and when it is automatically selected in CTO servers:

HP ML350 Gen9 X4 RPS Enablement Kit

NOTE: ML350 Gen9 supports max. four Power Supply slots. If Power supply selection qty is more than two, then X4 RPS Enablement Kit must be selected - to support the 3rd and 4th Power Supply Units. Minimum Power Supply qty. selection to configure with X4 RPS Enablement Kit is three. And this option is not supported with 720478-B21 (HPE 500W FS Plat Ht Plg Pwr Supply Kit). If hard drive cage selection qty. is more than three, this X4 RPS Enablement Kit must be selected - to get proper qty. of HDD backplane power connectors. If DW GPU card selection qty. is more than two, this X4 RPS Enablement Kit MUST be selected - in order to get proper qty. of GPU external power connectors.

***

HP ML350 gen9 User Guide:

http://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-c04431235-6.pdf

--> p. 126 where it specifically states:

x4 Redundant Power Supply backplane option

The x4 Redundant Power Supply backplane option must be installed when more that 3 SFF drive cages (including an NVMe Express bay) are used.

WARNING: When the x4 RPS kit is installed, at least three power supply modules must be installed in the server.

************

So the server was designed to work with either the x2 rps backplane and a minimum of 1 PSU (2 for redundancy) or the x4 rps backplane and a minimum of 3 PSUs (4 for redundancy).

I hope the information i provided was useful and clear enough.

[Nestorov]

there is also an advisory on the matter that says what my previous post contains, only with a lot less explanation:

http://h20565.www2.hpe.com/hpsc/doc/public/display?sp4ts.oid=7271259&docId=emr_na-c04912064&docLocale=en_US