You should understand the server I/Os and data mirroring that occur when a failure occurs on the quorum disk or when no volume is set for the quorum disk.
GAD pairs that meet the following requirements can continue operation using the S-VOL if the P-VOL is blocked:
- The option for setting no LDEVs for the quorum disk is enabled when the quorum disk is created.
- The pair is created using the quorum ID assigned when the quorum disk is created.
If the quorum disk is blocked, GAD pairs can keep the same data in the P-VOL and S-VOL, but the operation stops if the P-VOL is blocked. To continue the operation, you must delete the GAD pair.
Server I/Os for GAD pairs and GAD pair data mirroring
Server I/Os for GAD pair and GAD pair data mirroring are as follows:
- When the quorum disk is blocked and the pair status is PAIR or when no volume is set for the quorum disk and the pair status is PAIR The primary and secondary storage systems communicate through remote paths. Because the P-VOL and S-VOL pair status and the I/O mode remain PAIR (Mirror(RL)), server I/Os continue in the PVOL and the S-VOL. Data mirroring can be maintained through remote paths between the primary and secondary storage systems.
- When the quorum disk is blocked and the pair status is INIT/COPY Server I/Os continue in the P-VOL; however, that the pair might be suspended if the quorum disk is blocked immediately after the pair status changes to COPY.
- When no volume is set for the quorum disk and the pair status is INIT/COPY Server I/Os continue in the P-VOL.
- When the pair is suspended (pair status is PSUS, PSUE, SSWS, or SSUS) and the quorum disk is blocked or when the pair is suspended and no volume is set for the quorum disk Server I/Os continue in the volume with Local I/O mode. I/Os to the volume with Block I/O mode remain stopped, and data mirroring remains suspended.
- When the remote paths are disconnected after the quorum disk is blocked or when the remote paths are disconnected and no volume is set for the quorum disk after the quorum disk is blocked, the pair is suspended when the remote paths are disconnected or if no volume is set for the quorum disk. The P-VOL status and the I/O mode change to PSUE (Local), and the S-VOL status and the I/O mode change to PAIR (Block). Server I/Os continue in the P-VOL. The pair might be suspended and the status and the I/O mode of the P-VOL and the S-VOL might change to PSUE (Block) depending on the timing of the remote path disconnection after the quorum disk is blocked or if no volume is set for the quorum disk.
Before the pair status of the S-VOL and the I/O mode change to PAIR (Block), reading data might be delayed. If you want to minimize the delay, set a smaller value for Read Response Guaranteed Time When Quorum Monitoring Stopped in Storage Navigator. The time between the remote path disconnection and the pair suspension is also shortened.
When you want to restore the remote path quickly and do not want to suspend pairs immediately after the remote path is disconnected, set a larger value for Read Response Guaranteed Time When Quorum Monitoring Stopped in Storage Navigator. If you set a value larger than the server timeout time, a timeout might occur on the server.
The following table lists the recommended values for Read Response Guaranteed Time When Quorum Monitoring Stopped.
| Setting value for Blocked Path Monitoring (sec) | Recommended setting value for Read Response Guaranteed Time When Quorum Monitoring Stopped |
|---|---|
| 40 (Default) | 40 (Default) |
| 2 to 5 | 5* |
| 6 to 25 |
6 to 25* |
| 26 to 44 | 26 to 44 |
| 45 | 45 |
|
* A GAD pair might be suspended if remote path communication is blocked temporarily due to an MP or path failure. To avoid this, a value which is greater than the RIO MIH time or at least 25 seconds must be set for Read Response Guaranteed Time When Quorum Monitoring Stopped. Note, however, that reading data might delay up to the time set for Read Response Guaranteed Time When Quorum Monitoring Stopped. |
|
Setting the same value as the blocked path monitoring for Read Response Guaranteed Time When Quorum Monitoring Stopped is recommended. Until the pair status and I/O mode of the S-VOL change to PSUE (Block), delay of reading data can be maintained within the seconds set for Read Response Guaranteed Time When Quorum Monitoring Stopped. Note that if a value equal to or less than 5 seconds is set for the blocked path monitoring, you must set 5 for Read Response Guaranteed Time When Quorum Monitoring Stopped.
If a value equal to or greater than 46 seconds is set for Read Response Guaranteed Time When Quorum Monitoring Stopped, GAD pair suspension caused by a remote path failure might be avoided. When you set a value of 46 or greater, make sure that the application timeout setting for server I/Os is greater than this value. Also, make sure that multiple remote paths are set (at least four paths are recommended). Reading data might be delayed until the time set for Read Response Guaranteed Time When Quorum Monitoring Stopped elapses.