When you set up the TrueCopy association between the primary and secondary systems, you specify a time limit in milliseconds (ms) for data to travel from the P-VOL to the S-VOL, which is called the round trip (RT) time. 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 can even interrupt the initial copy operation.
An example of a significant difference is 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 insignificant, initial copying is allowed to continue at the specified pace.
An example of an insignificant 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.
RT time can be set between 1 ms and 500 ms, depending on the following scenarios:
The following equation lets you set the appropriate RT time, in 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.
|
Host mode option 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.
If the physical path between the primary and secondary storage systems is an iSCSI, the number of response sequence is determined in proportion to the initial copy speed because the transferred data is divided into 64 KB.
|
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 of the initial copy pace, the number of maximum initial copy, and the bandwidth of the channel extender communication lines between the primary and secondary systems.
| Initial copy response time equation |
|---|
|
Notes:
|
The following table shows the initial copy pace used in the initial copy response time equation.
|
Interface |
When executing initial copy only |
When executing initial copy operations and update copy at the same time |
||
|---|---|---|---|---|
|
When initial copy pace specified at the time of pair creation is 1 to 4 |
When initial copy pace specified at the time of pair creation is 5 to 15 |
When initial copy pace specified at the time of pair creation is 1 to 2 |
When initial copy pace specified at the time of pair creation is 3 to 15 |
|
|
Device Manager - Storage Navigator |
User-specified value |
4 |
User-specified value |
2 |
|
CCI |
User-specified value |
4 |
User-specified value |
2 |
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 |
4 |
64 |
160 |
|
30 |
0.1 |
4 |
4 |
64 |
220 |
|
100 |
0.1 |
4 |
4 |
64 |
360 |