IBM Flex System V7000 Expansion Enclosure 4939H29 Manual Do Utilizador

Draft Document for Review January 29, 2013 12:52 pm

IBM Flex System V7000 Storage Node Introduction and Implementation Guide

Global Mirror provides an asynchronous copy, which means that the secondary volume is not 
an exact match of the primary volume at every point in time. The Global Mirror function 
provides the same function as Metro Mirror Remote Copy without requiring the hosts to wait 
for the full round-trip delay of the long-distance link; however, some delay can be seen on the 
hosts in congested or overloaded environments. Make sure that you closely monitor and 
understand your workload.

In asynchronous Remote Copy, which Global Mirror provides, write operations are completed 
on the primary site and the write acknowledgement is sent to the host before it is received at 
the secondary site. An update of this write operation is sent to the secondary site at a later 
stage, which provides the capability to perform Remote Copy over distances exceeding the 
limitations of synchronous Remote Copy. 

The distance of Global Mirror replication is limited primarily by the latency of the WAN Link 
provided. Global Mirror has a requirement of 80 ms round-trip-time for data sent to the remote 
location. The propagation delay is roughly 8.2 µs per mile or 5 µs per kilometer for Fibre 
Channel connections. Each device in the path adds addition delay of about 25 µs. Devices 
that use software (such as some compression devices) add much more time. The time added 
by software-assisted devices is highly variable and should be measured directly. Be sure to 
include these times when planning your Global Mirror design. 

You should also measure application performance based on the expected delays before 
Global Mirror is fully implemented. The IBM Flex System V7000 Storage Node provides you 
with an advanced feature of Global Mirror that permits you to test performance implications 
before deploying Global Mirror and obtaining a long-distance link. This advanced feature is 
enabled by modifying the IBM Flex System V7000 Storage Node parameters 

 and 

. These two parameters could be used 

to simulate the write delay to the secondary volume. The delay simulation can be enabled 
separately for each intra-cluster or inter-cluster Global Mirror. You can use this feature to test 
an application before the full deployment of the Global Mirror feature.

Figure 9-61 on page 387 shows that a write operation to the master volume is acknowledged 
back to the host issuing the write before the write operation is mirrored to the cache for the 
auxiliary volume.