Determining the Round Trip Time

You specify a time limit for data to be transferred from the P-VOL to the S-VOL when you set up the GAD association between the primary and secondary storage systems. This value is the reference value to control the initial copy pace automatically and to lessen the impact to host response time of the remote I/O for the update I/O.

The default value of the Round Trip Time is 1 millisecond, and the range of the Round Trip Time is 1 ms to 500 ms. If the distance between the primary and secondary storage systems is large, or if a delay is caused by the line equipment, you need to change the Round Trip Time to a value that is appropriate for your environment. If the initial copy operation is performed with the default Round Trip Time value of 1 ms instead of a more appropriate value, it might take an unexpectedly long time to complete initial copy operations.

Note the following Round Trip Time considerations:

If the difference between the Round Trip Time and remote IO response time is significant, for example, 1 ms for Round Trip Time and 500 ms for remote I/O response time, the storage system slows or even interrupts the initial copy operation so that update copy can continue.

If the difference between the Round Trip Time and remote IO response time is insignificant, for example, 1 ms for Round Trip Time and 5 ms for remote I/O response time, the storage system allows initial copying to run at the specified pace.

To determine the Round Trip Time value


Round Trip time = round trip time between the primary and secondary storage systems × 2* + initial copy response time (ms)
* When host mode option (HMO) 51 (Round Trip Set Up Option) is OFF (default), you must double the round trip 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 round trip time, because the sequence is one response for each command issued.

For the "round trip time" in the formula, ask customer support, or use the ping command. If you are not using channel extenders between the primary and secondary storage systems, specify "1".

The "initial copy response time" in the formula is the response time required for multiple initial copy operations. Use the following formula to determine the initial copy response time based on the initial copy pace, the number of maximum initial copy VOLs, and the bandwidth of the channel-extender communication lines between the primary and secondary storage systems.


Initial copy response time (ms) = (1[MB] / "Data path speed between the primary and secondary storage systems [MB/ms]¹") × ("initial copy pace² / 4) × (number of maximum initial copy VOLs³ " / "Number of data paths between the primary and secondary storage systems"⁴)
When you connect the primary and secondary storage systems without extenders, set the value of the "data path speed between the primary and secondary storage systems" according to the link speed as follows: 2 Gbps: 0.17 MB/ms 4 Gbps: 0.34 MB/ms 8 Gbps: 0.68 MB/ms 10 Gbps: 0.85 MB/ms (VSP G1000 only) 16 Gbps: 1.36 MB/ms For "initial copy pace" in the preceding formula, see the following table. For "number of maximum initial copy volumes", use the value set up per storage system. The default value is 64. Even if the "number of maximum copy VOLs" / "Number of data paths between the primary and secondary storage systems" is 16 or more, specify "number of maximum initial copy VOLs" / "Number of data paths between the primary and secondary storage systems" as 16.

Initial copy response time (ms) = (1[MB] / "Data path speed between the primary and secondary storage systems [MB/ms]¹") × ("initial copy pace² / 4) × (number of maximum initial copy VOLs³ " / "Number of data paths between the primary and secondary storage systems"⁴)

When you connect the primary and secondary storage systems without extenders, set the value of the "data path speed between the primary and secondary storage systems" according to the link speed as follows:

2 Gbps: 0.17 MB/ms

4 Gbps: 0.34 MB/ms

8 Gbps: 0.68 MB/ms

10 Gbps: 0.85 MB/ms (VSP G1000 only)

16 Gbps: 1.36 MB/ms

For "initial copy pace" in the preceding formula, see the following table.

For "number of maximum initial copy volumes", use the value set up per storage system. The default value is 64.

Even if the "number of maximum copy VOLs" / "Number of data paths between the primary and secondary storage systems" is 16 or more, specify "number of maximum initial copy VOLs" / "Number of data paths between the primary and secondary storage systems" as 16.

The following table shows the initial copy pace used for the response time calculation.


Interface	Initial copy only in progress		Initial, update copy in progress
Interface	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
CCI	User-specified value at the time of pair creation	4	User-specified value at the time of pair creation	2
Device Manager - Storage Navigator	User-specified value at the time of pair creation	4	User-specified value at the time of pair creation	2

The following table shows example settings.


Round trip time[ms]	Data path speed between the primary and secondary storage systems (MB/ms)	Number of data paths between the primary and secondary storage systems	Initial copy pace	Number of maximum initial copy VOLs	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