RAID Controllers: Cache Modules, Batteries and Compatibility
RAID controllers are ordered wrongly more often than the drives attached to them, partly because the cache module and its power backup are separate parts that people assume are included.
The controller is frequently three parts
This is the first thing to understand before ordering.
The controller card itself.
The cache module, which on many designs is a separate component that plugs into the card rather than being soldered on.
The cache power backup — a battery or supercapacitor with a cable, which protects cache contents during a power loss.
Each has its own part number, and a controller bought without the cache module or without the power backup is not a like-for-like replacement.
That matters because a controller without working cache protection runs in write-through mode, and write performance drops sharply. Our controller guide covers why, and our performance guide covers why this is the first thing to check on a sudden write slowdown.
Battery or supercapacitor
Two approaches to the same problem, and they are not interchangeable.
Battery-backed cache holds cache contents powered until the system returns. Batteries degrade and need replacing periodically, and a failed battery drops the controller to write-through.
Flash-backed cache uses a supercapacitor to power a write of cache contents into non-volatile flash. No periodic battery replacement, and the supercapacitor still ages.
Two practical points.
The power backup is model-specific, including the cable, and cables differ between generations.
Check the status rather than assuming. The controller reports cache protection state, and a degraded or learning battery is reported rather than hidden — but only if someone looks. Our monitoring guide covers routing that where it will be noticed.
Controller mode changes what you should buy
Because the same physical card can serve two purposes.
RAID mode assembles arrays in hardware and presents logical volumes to the operating system.
HBA or IT mode passes drives through individually, which is what software storage layers need in order to see physical drives and their health data.
Not every card supports both, and some require a firmware change rather than a setting. Our controller guide covers the distinction, including why running a software storage layer on hardware-assembled arrays means both layers handling failures with different assumptions.
If you need passthrough, confirm the specific card supports it before buying — it is not a universal capability.
Port count and internal versus external
Three things to check that are easy to get wrong.
Internal, external, or both. Cards come in variants with internal connectors for a backplane, external connectors for an enclosure, or a mix.
Port and lane count, which determines how many drives can be attached directly.
Connector generation, since these changed over time and do not physically mate across generations. Our cables guide covers identifying them.
Note that cables are frequently not supplied with a controller, and the correct cable depends on the chassis rather than the card alone.
Moving arrays between controllers
Worth being careful about, because this is where data is lost.
Array metadata is written by the controller, and controllers of different families or firmware levels may not read each other’s arrays.
Three cautions.
Same family and similar firmware is usually required for an array to import cleanly.
Back up before attempting it, and verify the backup restores rather than trusting the job reported success. Our backup guide covers why an untested backup is an assumption.
Do not initialise anything when a replacement controller offers to. That is the action that turns a recoverable situation into a restore.
Identifying the controller you have
Three methods, in order of convenience.
Query the system. The management controller and the storage controller utility both report the model, firmware and cache configuration, including cache protection status. No need to open anything — our management guide covers access.
Read the card label during a maintenance window, and photograph the cache module and the power backup separately if they are present, since those carry their own part numbers.
Check the original configuration if you have it, remembering that it describes what shipped rather than what is fitted now.
What to send with an enquiry
The controller model and firmware level. The server model and generation. Whether you need cache and cache protection, or already have them. Whether you need RAID or passthrough. How many drives, internal or external. And whether the existing array must be preserved — because that changes the answer considerably.
Our quote guide covers what else speeds an enquiry up.
Common questions
Does a replacement controller include the cache module?
Frequently not. On many designs the cache module and its battery or supercapacitor are separate parts with their own part numbers, and a controller without working cache protection runs in write-through mode with sharply lower write performance.
Battery or flash-backed cache?
Battery-backed holds cache powered until the system returns and needs periodic replacement. Flash-backed uses a supercapacitor to write cache into non-volatile flash. Both age, both are model-specific including the cable, and both report their status.
Can I move an array to a different controller?
Usually only within the same family and similar firmware, since array metadata is controller-written. Back up and verify the restore first, and do not initialise anything when a replacement controller offers to — that turns a recoverable situation into a restore.
Does every controller support passthrough?
No. HBA or IT mode is not universal, and some cards need a firmware change rather than a setting. If a software storage layer needs to see physical drives and their health data, confirm the specific card supports it before buying.
How do I find out which controller I have?
Query the system — the management controller and storage utility report model, firmware and cache configuration including protection status, without opening anything. Photograph the cache module and power backup separately, since they carry their own numbers.
Tell us the controller model and whether cache and its backup are present, and we will confirm what you need before you order.
