When you set up the TrueCopy for Mainframe association between the primary and secondary storage systems, you specify a time limit in milliseconds for data to travel from the P-VOL to the S-VOL, which is called the round trip (RT) time. The RT time is used to control the initial copy pace while update copy operations are in progress.
- If the difference between the RT time you set and the remote I/O response time is significant, the storage system slows down or the initial copy operation is interrupted.
Example of a significant difference: 1 ms RT time and 500 ms remote I/O response time.
- If the difference between the RT time and the remote I/O response time is not significant, initial copying is allowed to continue at the specified pace.
An example of an significant difference is 1 ms RT time and 5 ms remote I/O response time.
- You can adjust the RT time when the distance between the primary and secondary systems is long, or when there is a delay caused by the line equipment. There can be a delay in completing the initial copy operation if it is performed with the default RT time instead of the appropriate value.
- The default RT time is 1 ms.
The RT time can be set between 1 ms and 500 ms, depending on the following scenarios:
Use the following equation to set the appropriate RT time (ms):
RT-time = RT-time-between-the-primary-and-secondary-storage-systems × number-of-responses + initial-copy-response-time (ms)
If the physical path between the primary and secondary storage systems uses Fibre Channel technology, the number of responses depends on the host mode option (HMO) 51 setting, as shown in the following table.
|
HMO 51 |
Number of responses |
|---|---|
|
OFF |
2 |
|
ON |
1 |
When HMO 51 is OFF (default), you must double the RT time because each data transfer between the primary and secondary storage systems involves two response sequences for each command issued.
When HMO 51 is ON, you do not need to double the value of the RT time, because the sequence is one response for each command issued.
|
Initial copy speed |
Number of responses |
|---|---|
|
1 |
6 |
|
2 |
10 |
|
3 |
14 |
|
4 |
18 |
- Use the ping command when setting the RT time, or contact customer support. If you do not use channel extenders between the primary and secondary systems, specify "1".
- The initial-copy-response-time is the response time required for multiple initial copy operations.
Use the following equation to determine the initial copy response time by using the initial copy pace, the maximum number of initial copy volumes, the data path speed between primary and secondary systems, and the number of data paths between primary and secondary systems.
Initial-copy-response-time (ms) = (1 (MB)/data-path-speed-between-primary-and-secondary-storage-systems (MB/ms)1 × (initial-copy-pace2/4) × (maximum-initial-copy-activities3/number-of-data-paths-between-primary-and-secondary-storage-systems4)
Notes:
- When you connect the primary storage system and secondary storage system without channel extenders, set the data path speed between the primary and secondary storage systems to one of the following values according to link speed:
- 4 Gbps: 0.34 MB/ms
- 8 Gbps: 0.68 MB/ms
- 16 Gbps: 1.36 MB/ms
- 32 Gbps: 2.72 MB/ms
- 64 Gbps: 5.44 MB/ms
- For details about initial-copy-pace, see the next table.
- For maximum-initial-copy-activities, use the value set up per storage system. The default is 64.
- Even if the maximum initial copy activities or the number of maximum-initial-copy-activities/number-of-data-paths-between-primary-and-secondary-storage-systems is larger than 16, specify it as 16.
The following table shows the initial copy pace used in the initial copy response time equation.
|
Interface |
Initial copy only in progress |
Initial, update copy in progress |
|
|---|---|---|---|
|
When initial copy pace specified at the time of pair creation is 1 to 8 |
When initial copy pace specified at the time of pair creation is 9 to 15 |
||
|
Business Continuity Manager |
User-specified value |
User-specified value between 1 and 8 |
8 |
|
PPRC command (TSO/ICKDSF) and CU emulation is I-2107 |
15* |
8* |
|
|
* When a PPRC command is used and the CU emulation type is I-2107, the initial copy pace specified is invalid. Therefore, the fixed value will be set. |
|||
The following table shows examples for RT time settings for multiple initial copy operations.
|
Round trip time between primary and secondary system (ms) |
Data path speed between primary and secondary systems (MB/ms) |
Number of data paths between primary and secondary systems |
Initial copy pace |
Maximum initial copy activities |
Round trip time specified (ms) |
|---|---|---|---|---|---|
|
0 |
0.1 |
4 |
15 |
64 |
160 |
|
30 |
0.1 |
4 |
15 |
64 |
220 |
|
100 |
0.1 |
4 |
15 |
64 |
360 |