Working in the Hot Aisle: Power and Cooling

Part 1 of this saga can be found here.

An hour of battery backup is plenty of time to shut down a dozen servers… until it isn’t.  The last thing you want to see is that clock ticking down while a couple thousand Windows virtual machines decide to install updates before shutting down.

We were fortunate that one of the offices we were consolidating was subleased from a manufacturer who had not only APC in-row chillers that they were willing to sell, but a lightly used generator.  Between those and a new Symmetra PX UPS, we were on our way to breathing easy when the lights went out.  The PX provides several hours of power and is backed by the generator, which also backs up the chillers in the event of a power outage.  The PX is a marvel of engineering, but is also a single point of failure.  We witnessed this firsthand with an older Symmetra LX, which had a backplane failure a couple of years previous that took down everything.  With that in mind, we opted to go with two PDUs in each server cabinet: one that fed from the UPS and generator, and one that fed from city power with a massive power conditioner in front of it.  These circuits also extend into the IDFs so that building-wide network connectivity stays up in the event of a power issue.

Most IT equipment comes with redundant power supplies, so splitting the load is easy: one power supply goes to each PDU.  For the miscellaneous equipment with a single power supply, an APC 110V transfer switch handles the switching duties.  A 1U rackmount unit, it is basically a UPS with two inputs and no batteries, and it seamlessly switches from one source circuit to the other when a voltage drop is detected.

As mentioned, cooling duties are handled by APC In-Row chillers, two in each aisle.  They are plumbed to rooftop units and are backed by the generator in case of power failure.  Temperature sensors on adjacent cabinets provide readings that help them work as a group to optimize cooling, and network connectivity allow monitoring using SNMP and/or dedicated software.  Since we don’t yet need the cooling power from all four units, we will be programming the units to run on a schedule to balance running hours between the units.

Cooling in the IDFs is handled by the building’s chiller, with an independent thermostat-controlled exhaust fan as backup. As each IDF basically hosts just one chassis switch, cooling needs are easily handled in this manner. As users are issued laptops that can ride out most outages, we were able to sidestep having to provide UPS power to work areas.

Next time:  Keeping the hot hot and the cool cool.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.