HP XP7 Storage - Manual (Page 31)

System performance considerations
Synchronous copy operations affect I/O performance on the host and on the primary and secondary
systems. Continuous Access Synchronous Z provides options for minimizing the impact and for
maximizing the efficiency and speed of copy operations for the best level of backup data integrity.
These options are discussed in:
•
“Determining Round Trip Time” (page 31)
•
“Determining Minimum Number of Remote Paths” (page 32)
•
“Allowing I/O to the P-VOL after a split—Fence Level options” (page 36)
Performance is also optimized with the proper bandwidth. This is discussed in “Analyzing workload,
planning data paths” (page 32) .
Determining Round Trip Time
You specify a time limit in milliseconds for data to travel from the P-VOL to the S-VOL when you
set up the Continuous Access Synchronous Z association between primary and secondary system.
Round Trip Time is used by the systems to control the initial copy pace when update copying is in
progress.
This section provides instructions for determining your system’s Round Trip Time.
Note the following Round Trip Time considerations:
•
If the difference between Round Trip Time and remote IO response time is significant, the
system slows or even interrupts the initial copy operation so that the update copy can continue.
Example of significant difference between the two: 1ms RT Time : 500ms remote IO response
time.
•
If the difference between the two is insignificant, initial copying is allowed to run at the specified
pace.
Example of insignificant difference between the two: 1ms : 5 ms
To determine Round Trip Time value
The value of Round Trip time= round trip time between MCU-RCU x 2(*) + initial copy response time (ms)
(*) A data transfer between primary and secondary system involves two response sequences in one command issuance.
Therefore, you need to double the round trip time.
•
For the “round trip time” in the formula, please ask your service provider, or use a ping
command. If you do not use channel extenders between the primary and secondary system,
specify “1”.)
•
The “initial copy response time” in the formula is the response time required for multiple initial
copy operations. With the following formula, determine the initial copy response time using
the initial copy pace, the number of maximum initial copy VOLs, and the bandwidth of the
channel extender communication lines between primary and secondary system.
Initial copy response time (ms) = (1[MB] / “Data path speed between MCU-RCU[MB/ms]
1
”) x (“initial copy
pace(
2
/ 15) x (number of maximum initial copy VOLs
3
” / “Number of data paths between MCU-RCU”
4
)
1. When you connect MCU with RCU without channel extenders, specify 0.17[MB/ms] in “Speed of line between
MCU and RCU”.
2. For “initial copy pace” in the preceding formula, see the following table.
3. For “number of maximum initial copy volumes”, use the value set up per storage system. The default is 64.
4. Even if the “number of maximum copy VOLs” / “Number of data paths between MCU-RCU” is larger than
16, specify “number of maximum initial copy VOLs” / “Number of data paths between MCU-RCU” as 16.
System performance considerations 31