Dell PowerVault ME5 Series Administrator's Guide

Using write-back or write-through caching

You can change the cache policy setting of a volume in a dual-controller system.

Write-back is a cache-writing strategy in which the controller receives the data to be written to disks, stores it in the memory buffer, and immediately sends the host operating system a signal that the write operation is complete, without waiting until the data is written to the disk. Write-back cache is saved to nonvolatile storage in the event of a power loss. Data in the cache is mirrored from one controller module cache to the other. If a controller fails, the remaining controller completes the write operation to the disks. Write-back cache improves the performance of write operations and the throughput of the controller.

In write-through caching mode, the controller writes the data to the disks before signaling the host operating system that the process is complete. Write-through cache has lower write throughput performance than write-back, but it is the safer strategy, with minimum risk of data loss on power failure. Write-through cache does not require mirroring the write data because the data is written to the disk before signaling that the command is complete. You can set conditions that cause the controller to change from write-back caching to write-through caching.

In both caching strategies, active/active failover of the controllers is enabled.

You can enable and disable the write-back cache for each volume:

• In a dual-controller system, with both controllers running, volume write-back cache is enabled by default. Because controller cache is backed by supercapacitor technology, if the system loses power, data is not lost. For dual-controller systems, write-back is the preferred setting.
• In a single-controller system, or if a controller fails in a dual-controller system, write-through cache is enabled by default.