The write cache is a feature of some RAID controllers that allows them to temporarily store data in a fast memory buffer before writing it to the disk drives. This improves the performance and efficiency of write operations, especially for random and small writes. However, if the battery on the controller goes bad, the controller may disable the write cache to prevent data loss in case of a power failure. This can degrade the disk drive performance significantly, as every write operation will have to wait for the disk drives to complete.
References:
https://www.dell.com/support/kbdoc/en-us/000131486/understanding-raid-controller-battery-learn-cycle
https://www.techrepublic.com/article/understanding-raid-controller-write-cache/