Managing RAID on Linux

Write Cache

Controller cache memory operates in one of two modes. Each mode offers trade-offs between performance and reliability.

Write-back caching

When the controller is configured for write-back caching, the controller holds data in the controller’s memory until it is full or until the controller is idle and then commits the data to disk. This mode yields the best performance, but it’s not as reliable as write-through caching because a system failure could result in the loss of data that is still in the controller’s memory, but not flushed to disk.

Having a controller battery is very helpful when using the write-back caching method because a power failure only means that unflushed buffers are stored until the system restarts. Extra controller memory is also important for write-back caching. On heavily used systems with lots of sequential disk I/O, it’s a good idea to consider getting a memory upgrade.

Write-through caching

Write-through caching commits data to disk immediately. This method is much slower than write-back caching, but it ensures that all writes are committed to disk and are never lost because a failure occurred while unwritten buffers were waiting in the controller’s memory. If you use write-through caching, the amount of memory on your controller is not as important as getting a fast controller and fast disks.

Each array can generally use its own caching method. Thus, it’s possible to configure heavily used, less important arrays for write-back caching, and critical, ...