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A Neutral Device to Maintain Storage Mirror Synchronization States

IP.com Disclosure Number: IPCOM000127247D
Original Publication Date: 2005-Aug-18
Included in the Prior Art Database: 2005-Aug-18
Document File: 1 page(s) / 21K

Publishing Venue

IBM

Abstract

Storage is often mirrored between two storage devices for reliability. The two devices communicate with each other over some network connection to maintain synchronization between the two devices. However, if that network connection fails or a partner device is off-line then a storage device is unable to maintain the synchronization of its partner, and the partner is unable to know if it is still synchronized. All storage accesses must be rejected at that point. Our invention provides an out-of-band communication method between the two mirrored devices through a third neutral device that maintains the synchronization state of the two devices in the event that one of the devices is off-line or the normal network connection between the two devices is broken. The out-of-band communication is over a low bandwidth connection that may be available in the event of a failure of the normal network connection.

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A Neutral Device to Maintain Storage Mirror Synchronization States

     Our invention is used in a system containing two storage devices that need to be synchronized. Our invention places a third device in the system that has an out-of-band communication path to both storage devices and maintains the synchronization state of the two storage devices.

     The third device has communication to both of the storage devices through a path that is not the normal communication path between those devices. This out-of-band path supports limited functionality, and, therefore, could be of lower bandwidth than the normal path. The third device maintains two boolean flags in persistent storage; one for each storage device (or logical storage device). A true value indicates that the device is synchronized, and a false value indicates not synchronized. When the two storage devices are able to communicate with each other over the normal path, they perform the necessary steps to synchronize themselves and then write the appropriate flag in the third device to true over the out-of-band path. As long as the normal path is connected, the two storage devices keep each other synchronized using the normal path.

     When the normal path breaks or a storage device goes off-line, then the storage devices are unable to keep their partners synchronized. When a storage device receives an update request that it is unable to pass to its partner, the storage device will change the flag in the third device fo...