Dell PowerVault ME5 Series Administrator's Guide

Data protection during failover to a single controller

The system can operate with a single controller if its partner has gone offline or has been removed but this configuration no longer provides redundancy.

When the ME5 system fails over to a single controller, the system switches to write-through caching mode. In write-through mode, the controller waits to notify the host that it has received a write until after the data is written to disk. This configuration guards against data loss during the failover.

If you set the caching policy to write-back mode, data to be written is held in the NVM cache. If the remaining controller fails while in write-back mode, unwritten cache data likely exists. The same is true if the controller or the enclosure of the target volume is powered off without a proper shutdown. Data remains in the controller cache and associated volumes will be missing that data on the disk.

• If the controller can be brought back online long enough to perform a proper shutdown and the disk group is online, the controller should be able to write its cache to disk without causing data loss.
• If the controller cannot be brought back online long enough to write its cache data to disk, contact technical support.

For more information about volume cache options, see Volume cache options. To edit volume cache options, select Provisioning > Volumes and view the volume slide-over panel.