Dell SRDF Introduction

Invalid tracks are tracks that are not synchronized between the two devices in an SRDF pair. They occur when either member of the pair cannot communicate with its partner; for example, due to a failure of the SRDF link between the storage arrays. On both sides of the configuration, the storage arrays record the number of tracks that are owed to the other side.

Once the two devices can communicate once again, the invalid tracks need resolving between the pair. There are two ways to resolve the tracks:

• Copy the modified R1 tracks to the R2 side. Any tracks that were modified on the R2 side are overwritten with the data for the corresponding tracks on the R1 side.
• Copy the modified R2 tracks to the R1 side. Any tracks that were modified on the R1 side are overwritten with the data for the corresponding tracks on the R2 side.

Example: SRDF link or R2 device is unavailable

Here, the SRDF link is unavailable for some reason, the R2 device is unavailable, or both the link and the R2 device are unavailable. The R1 device, however, remains write accessible to the application host. While this situation exists, the R1 device receives I/O from the application host, and invalid tracks accumulate on the R1 array.

Once the SRDF link and the R2 device are available again, the array containing the R1 array sends the invalid tracks to the R2 device so that the two devices are synchronized once more.

Example: R1 is unavailable

Here, the R1 device has become unavailable for some reason. To maintain service to the application host, processing is moved to the R2 device. That is, the R2 device is made write accessible to the application host, and it receives I/O from that host. While this situation exists, invalid tracks accumulate at the R2 array.

Once the R1 device is available again, the array containing the R2 device sends the invalid tracks to the R1 device. Once the two devices are fully synchronized, processing returns to the R1 device and the R2 device is made write protected to the application host.