Manual Pages  — DA

The read cache is used to store data from device-initiated read ahead operations as well as frequently used data. The read cache is transparent to the user and can be enabled without any adverse effect. Most devices with a read cache come from the factory with it enabled. The read cache can be disabled by setting the RCD (Read Cache Disable) bit in the caching control mode page.

The write cache can greatly decrease the latency of write operations and allows the device to reorganize writes to increase efficiency and performance. This performance gain comes at a price. Should the device lose power while its cache contains uncommitted write operations, these writes will be lost. The effect of a loss of write transactions on a file system is non-deterministic and can cause corruption. Most devices age write transactions to limit vulnerability to a few transactions recently reported as complete, but it is none-the-less recommended that systems with write cache enabled devices reside on an Uninterruptible Power Supply (UPS). The da device driver ensures that the cache and media are synchronized upon final close of the device or an unexpected shutdown (panic) event. This ensures that it is safe to disconnect power once the operating system has reported that it has halted. The write cache can be enabled by setting the WCE (Write Cache Enable) bit in the caching control mode page.