Dell PowerVault ME4 Series Storage System Administrator’s Guide

About data protection with a single controller

The system can operate with a single controller if its partner has gone offline or has been removed. Because single-controller operation is not a redundant configuration, this section presents some considerations concerning data protection.

The default caching mode for a volume is write back, as opposed to write through. In write-back mode, the host is notified that the controller has received the write when the data is present in the controller cache. In write-through mode, the host is notified that the controller has received the write when the data is written to disk. Therefore, in write-back mode, data is held in the controller cache until it is written to disk.

If the controller fails while in write-back mode, unwritten cache data likely exists. The same is true if the controller enclosure 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, please contact technical support.

To help prevent data loss in case the controller fails, you can change the caching mode of a volume to write through. While this will cause significant performance degradation, this configuration guards against data loss. While write-back mode is much faster, this mode is not guaranteed against data loss in the case of a controller failure. If data protection is more important, use write-through caching. If performance is more important, use write-back caching.

For more information about volume cache options, see About volume cache options. For more information about changing cache settings for a volume, see Modifying a volume. For more information about changing system cache settings, see Changing system cache settings.