[saltcured]

This is how battery-backed RAID cards used to work. An on-card rechargeable battery could keep the write buffers alive for about 24-48 hrs to flush to disk after you restored power in your datacenter.

Now, they use super-capacitors and a flash memory device to flush the volatile RAM buffers to flash memory, then on next power up the firmware can flush the flash buffers to disk. This all happens within the RAID controller so the host OS just thinks it already got write confirmation back when the PCIe transactions finished and the data blocks landed in the controller's RAM.