What is a BBU?
A BBU (battery backup unit) is a battery module that keeps a component's memory or power alive when mains power fails. In PC and server hardware you'll usually meet it on a hardware RAID controller, where it protects the controller's cache. The same name is used more loosely for other battery backup hardware, such as UPS battery modules.
Why a RAID controller needs one
Many hardware RAID controllers have onboard cache memory. In write-back mode the controller tells the operating system a write is finished as soon as the data is in that cache, then writes it to the drives a moment later. That's much faster, but the cache is volatile: if power fails before the data reaches the disks, those writes are lost and the array can end up corrupted.
The BBU powers the cache during an outage so the pending data survives. When power returns, the controller writes the cached data out to the drives. A battery can only hold the cache for a limited time, so the server needs power back before it runs flat.
Cache protection options
| Method | How it works | Upkeep |
|---|---|---|
| BBU (battery) | Keeps the cache memory powered until mains power returns | Batteries wear out and need replacing |
| Flash-backed cache | A supercapacitor powers the controller just long enough to copy the cache to onboard flash | No battery to replace; common on newer controllers |
| No protection | The controller should use write-through mode, writing straight to disk | Slower writes, but no data held only in cache |
Maintaining and troubleshooting a BBU
- Controller management tools report battery health, and a failed or degraded BBU is a common server alert.
- When the battery is missing, charging or failed, many controllers switch from write-back to write-through by default to protect data. The symptom is a sudden drop in write performance.
- Many controllers also run a periodic battery learn cycle and may drop to write-through while it runs.
- Forcing write-back without a working BBU is risky: a power cut can lose or corrupt data.
- A BBU is not a UPS. It protects only the controller's cache, and the server still goes down when power fails. Many servers use both.
In telecom, BBU also means a baseband unit, the signal-processing part of a cellular base station. That's unrelated to PC hardware.
For the CompTIA A+ exam
BBU isn't on the Core 1 (220-1201) acronym list, but it connects topics that are: RAID levels under objective 3.4, RAID troubleshooting under objective 5.2 (which lists RAID failure and low IOPS performance), and the UPS, which is on the acronym list. If a RAID server's writes suddenly slow down, a failed controller battery that forced write-through mode is one possible cause.