Round-trip response time is a time limit that is set for copying data from the P-VOL to the S-VOL. Round-trip response time is used as a reference value to automatically adjust the speed of initial copy when an initial copy operation is performed so the impact on the response time of remote I/O for update I/O is minimized.
By default, round-trip response time is 1 ms. However, change round-trip response time accordingly if the distance between the primary storage system and the secondary storage system is great or delays occur due to line devices. If an initial copy operation is performed without an adequate round-trip response time, the operation might take an unreasonably long time until it is completed. To set round-trip response time, use the raidcom modify rcu command with the -rcu_option option.
If the difference between the remote I/O response time and round-trip response time is large, for example, the remote I/O response time is 500 ms and round-trip response time is 1 ms, decrease the copy speed or temporarily stop the initial copy operation to prevent the initial copy operation from occupying the entire line bandwidth.
If the difference between the remote I/O response time and round-trip response time is small for example, the remote I/O response time is 5 ms and round-trip response time is 1 ms, perform the initial copy operation at the specified copy speed.
You can specify a value between 1 ms and 500 ms for round-trip response time. Use the following formula to determine the required length of round-trip response time:
round-trip-response-time-value (ms) = round-trip-time-between-primary-and-secondary-storage-systems x number-of-responses* + initial-copy-response time (ms)
* If the physical path connecting the primary and secondary storage system uses Fibre Channel, the number of responses depends on the setting of host mode option 51 (Round Trip Set Up Option).
| HMO 51 | Number of responses |
|---|---|
| OFF | 2 |
| ON | 1 |
When data is transferred, 2 responses are returned for each command, which means the number of responses is 2. When HMO 51 is set to ON, 1 response is returned for each command, which means the number of responses is 1. If the physical path connecting the primary and secondary storage system uses iSCSI, the number of responses is proportional to the initial copy speed, because data is divided into units of 64 KB when it is transferred.
| Initial copy speed | Number of responses |
|---|---|
| 1 | 6 |
| 2 | 10 |
| 3 | 14 |
| 4 | 18 |
To determine the round trip time between the primary and secondary storage systems, contact your telecommunications carrier or use the ping command to measure the time. If you connect the primary and secondary storage systems without using a telecommunications line, the number of responses is 1.