OVHcloud Data Centre Fire: How Its Cheapest Design Lost Everything

Key takeaways

• Fire broke out in SBG2 at about 00:35 on 10 March 2021, destroying the building and taking an estimated 3.6 million websites offline.
• Built in 2011 without mechanical chillers, SBG2 used the chimney effect for free cooling, and that open air column carried the fire upward.
• France’s BEA-RI report, dated 24 May 2022, named four factors in the spread: the building design and proximity, no automatic extinguishing system, too little water for firefighters and a delayed power cut.
• OVH cut power to the whole Strasbourg campus, including SBG1, SBG3 and SBG4, even though none of those buildings were burning.
• In March 2023 a French court ordered OVHcloud to pay damages over lost backup data stored inside the same campus.

Firefighters converging on OVHcloud’s Strasbourg campus that night faced a six-level tower that would not stop breathing. SBG2, the company’s most cost-efficient data centre, drew cool air in at its base and pushed hot air out at the roof by design, and that same design was now pulling a fresh draft of oxygen up through the fire, floor by floor. The first alarm had sounded at 00:35 local time on 10 March 2021. The OVHcloud data centre fire would destroy SBG2, damage its neighbour SBG1, and put an estimated 3.6 million websites offline within hours, according to wire coverage at the time.

Inside the OVHcloud data centre fire: a tower built to breathe

SBG2 had been built in 2011 around a specific bet: skip the chillers. Most data centres cool servers with mechanical refrigeration, and their efficiency is measured by PUE (power usage effectiveness, the ratio of total facility power to the power that reaches the servers). On a chiller-heavy site a large share of the electricity goes to cooling. OVH used a free cooling data centre design instead, auto-ventilation in its own terminology, built on the chimney effect: hot air is less dense than cold air, so it rises and exits at the roof on its own, pulling cool outside air in behind it without a compressor doing any of the work. That pushed OVH’s PUE down toward the floor and let it undercut chiller-dependent rivals on price across Europe. Operators still chase the same efficiency logic when they redesign 800V DC power distribution to cut copper losses in AI racks: cheaper by design, with a constraint that stays invisible until it fails.

The tower shape was the mechanism. Floor to roof, the building needed one continuous open air column for the physics to work at all, and nine years later that same column let a fire travel upward through the whole structure instead of staying in one room.

The OVHcloud data centre fire, hour by hour

Investigators and the trade press later reconstructed the sequence. The table sets each step against what was known at that point, which is often much less than what was concluded a year or two afterward.

Time	What happened	What was known then
2011	SBG2 built as a multi-storey free-cooling “tower,” no mechanical chillers	Marketed as OVH’s cost and energy efficiency edge
Days before 10 March 2021	Maintenance on ASI2, an inverter in SBG2’s energy room 2, after recurring unexplained switches to bypass mode	Intervention and restart-check reports mention nothing unusual
~00:35, 10 March 2021	Fire breaks out in SBG2, starting in two of its energy rooms, at an inverter and at the batteries connected to it	Cause unknown
Overnight to dawn, 10 March 2021	SDIS Bas-Rhin fights the blaze; SBG2 destroyed, part of SBG1 damaged; OVH cuts power to the entire Strasbourg campus, SBG1, SBG3 and SBG4 included	Customers told to activate disaster-recovery plans; full scale of damage still unclear
16 March 2021	OVH hands the damaged electrical equipment to police investigators	Cause an open criminal and technical inquiry
Late March 2021	OVH publishes a restoration roadmap, commits to rebuilding with a revised fire-safety design	Read at the time as OVH’s own admission the original design needed to change
24 May 2022	France’s BEA-RI dates its technical investigation report; The Register covers it on 10 June 2022	Report stops short of a definitive cause but lists four factors in the fire’s spread
March 2023	A French court orders OVHcloud to pay damages over lost backup data	Confirms that a backup hosted inside the same campus was not real redundancy

Why the gas never had a chance

Standard data centre fire suppression keeps water off live electrical equipment. It uses clean-agent gas (FM-200, Novec 1230, or an inert-gas blend) flooded into a sealed room to a specific design concentration and held there long enough to starve the fire of oxygen. The system only works if the room keeps the gas in for that time. SBG2 had no automatic fire-extinguishing system, and the 2022 BEA-RI report (from France’s Bureau d’Enquêtes et d’Analyses sur les risques industriels) later listed that among the four factors behind the fire’s spread, as set out in the BEA-RI report itself and The Register’s coverage of the investigation.

I’d call that an incompatibility more than an oversight. A building engineered to exchange air continuously from floor to roof cannot also hold a stable gas concentration in a sealed room. Put the chillers back and you could seal the room and gas it. Skip the chillers, as OVH did, and you also give up the option to smother a fire in its first minutes.

Four factors behind the spread

The OVH BEA-RI report stopped short of naming a definitive cause. It traces the start to an electrical fault at an inverter and at the batteries connected to it, in two energy rooms at almost the same moment, and says the cause of that failure cannot be established at this stage. It lists the hypotheses instead: liquid or humidity from the cooling system nearby, a malfunction linked to a maintenance operation the report describes as carried out “le matin même” (that same morning), without giving a date, and the inverter running outside its normal operating range. It is more confident about what let a single ignition become a six-level write-off, and it lists four factors: the design of the buildings and how close they stood to one another, the absence of any automatic extinguishing system, an insufficient water supply for firefighters, and the delay in making the site electrically safe.

The power-cut delay matters because crews cannot hose down live electrical equipment. They have to wait for confirmation that circuits are dead before fighting a fire at close range, and the fire burns unopposed for every minute of that wait. It’s the same electrical dependency that makes grid interconnection such a binding constraint for new data centre capacity on an ordinary day. The order to cut power came at 00:58, but the report puts the site as electrically safe only from about 02:30. There was no easily reached general cut-off, so the grid operator had to cut supply at the source substation, the backup generators were set to start on their own, and the charged UPS (uninterruptible power supply) batteries kept feeding power for an estimated twenty minutes after that. Water was the other limit. Crews had a single hydrant delivering less than 60 m³/h, and OVH had no water reserve of its own and no way to pump from the Rhine canal, until the fireboat EUROPA arrived at 03:00. The report does not blame the lead-acid batteries themselves for the spread; it describes the chemistry as more robust and more stable than lithium.

A backup in the same postcode is not a backup

OVH’s response that night went beyond the burning building. It cut power to the entire Strasbourg campus, including SBG1, SBG3 and SBG4, none of which were on fire, as a precaution while the blaze was fought next door. That is what site-level blast radius looks like in practice, as DataCenterDynamics later laid out in its analysis: three buildings went dark because a fourth one, sharing their electrical infrastructure and their address, was burning.

Some customers had paid for a backup option hosted inside that same campus, and for them the difference between a backup and a copy mattered more than the marketing had suggested. A copy sitting one building over shares every risk the original has: the same substation, the same site, in this case the same fire. A class action brought by more than 100 affected companies, later reported near 140, sought roughly €10 million in damages, and in March 2023 a French court ordered OVHcloud to pay damages over lost backup data. Redundancy that survives a building has to be somewhere else on the map.

The tradeoff nobody priced in

Most coverage of the OVHcloud data centre fire settled on a familiar moral: a cloud provider had a bad night, and customers who skipped backups paid for it. The report doesn’t support that reading. Two of BEA-RI’s four factors trace back to one decision made in 2011. The report puts building design first: SBG2 was built to move outside air through openings in its facade, and within 15 minutes of the fire starting smoke detectors had tripped on every floor, which the report reads as a building highly permeable to outside air and to smoke. The missing extinguishing system is harder to separate from that airflow, for the reasons above, though the report notes OVH fitted none of its five Strasbourg buildings with one. The other two, a single weak hydrant and a power cut that took until 02:30 to make safe, are site failures that a chiller-cooled building would have shared.

The free-cooling design that made SBG2 the cheapest large hosting a company could put a website on in Europe is the same open floor-to-roof air path that a sealed-room gas system needs to be absent. One engineering tradeoff was priced into the product as efficiency and never priced in as risk. OVHcloud rebuilt with a revised fire-safety design afterward, which is itself an admission that the original one needed to change.

Every large operator pushing PUE lower is making some version of the same bet, whether that means fighting a local zoning fight just to get a permit to build or arguing over where the megawatts come from at all. A room still needs a seal to hold a fire back. Take the seal away to save on chillers and you have changed what a fire means for that building, which is a much bigger step than a compromise at the margins.