You can reduce the time to recover from suspended GAD pair operations by forcibly changing the I/O mode of GAD pairs. Recovery time is reduced because recovery can be done by resynchronization instead of requiring that you re-create the GAD pairs.
Because of the serious problems that these operations might cause, perform these operations only when the following use cases can be applied:
- Due to a failure in the storage system, the pair status is PSUE and the I/O mode is Block for both the P-VOL and S-VOL, and operations are stopped.
- Operations are stopped due to a failure in a storage system. The P-VOL, or S-VOL, in the failure site requires time for recovery because the pair status and I/O mode cannot be checked. The S-VOL, or P-VOL, is suspended with I/O mode Block, so operations cannot resume.
- Operations are stopped due to a failure in a storage system. The P-VOL, or S-VOL, in the failure site, suspended with I/O mode Local, cannot be accessed due to a storage system failure, and requires time for recovery. The S-VOL, or P-VOL, is suspended with I/O mode Block, so operations cannot resume.
You can forcibly change the I/O mode of a suspended GAD pair in one of two ways, change the I/O mode from Block to Local or change the I/O mode from Local to Block. The following tables describe when you can forcibly change the I/O mode by using CCI, given the pair status, I/O mode, and the change that you want to perform.
For CCI:
| Pair status | I/O mode | Is the operation available | |
|---|---|---|---|
| Switching the I/O mode from Block to Local | Switching the I/O mode from Local to Block | ||
| PAIR | Any | No | No |
| COPY | Any | No | No |
| PSUS | Local | Yes (no status change) | Yes |
| Block | Yes | Yes (no status change) | |
| SSUS | Local | Yes (no status change) | Yes |
| Block | Yes | Yes (no status change) | |
| SSWS | Local | Yes (no status change) | Yes |
| Block | Yes | Yes (no status change) | |
| PSUE | Local | Yes (no status change) | Yes |
| Block | Yes | Yes (no status change) | |
| SMPL | Any | No | No |
Change of I/O mode in a GAD+UR environment
- After the I/O mode of the S-VOL of a GAD pair is switched from Block to Local, if the I/O mode of all pairs in the consistency group is switched, delta resync is automatically performed. For details about delta resync, see Executing delta resync.
- If an automatically performed delta resync does not end normally, the status of the UR pair becomes PSUE and, because the status is PSUE, you need to recover the UR pair by resynchronizing it. The initial copy that is performed during resynchronization copies the entire data, which is the same operation as when a pair is created. For details, see In case of delta resync failure.