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Chapter 18. Global Copy options and configuration 207
18.1.4 Terminate Global Copy pair
This operation ends the remote copy relationship between the volume pair; the volumes
return to the simplex state.
18.1.5 Convert a Global Copy pair to Metro Mirror
This operation is known as the Go-to-sync operation. There are two common situations when
you would convert a pair from Global Copy mode to Metro Mirror mode:
Situation 1
You have used Global Copy to complete the bulk transfer of data in the creation of many
copy pairs, and you now want to convert some or all of those pairs to Metro Mirror mode.
Situation 2
You have Global Copy pairs for which you want to make FlashCopy backups on the
recovery site. You convert the pairs temporarily to Metro Mirror mode in order to obtain
point-in-time consistent copies.
The transition of a volume pair from Global Copy to Metro Mirror is also referred to as the
catch-up transition, and it is discussed in the following section.
18.2 Catch-up transition
Copy Pending catch-up is the name of the transition for a Global Copy pair that goes from its
normal out-of-sync condition to a fully synchronous condition, that is, a full duplex Metro
Mirror pair. The pair goes from the
pending state to the duplex state. At the end of this
transition, primary and secondary volumes are fully synchronized, with all their respective
tracks identical.
As a result of the catch-up, the Global Copy pair becomes a Metro Mirror pair. This has an
impact on application response times because write I/Os in a Metro Mirror pair are written to
both primary and secondary volumes before the host application is acknowledged. For this
reason, a Global Copy pair is normally synchronized only for short periods of time, typically to
make a FlashCopy, and host application updates are quiesced for the duration. Refer to 18.3,
“Create a consistent point-in-time copy” on page 209 for a more detailed discussion.
18.2.1 Go-to-sync using TSO
You can initiate a catch-up operation by commanding Global Copy to go-to-SYNC with the
TSO command CESTPAIR. For the go-to-SYNC, the CESTPAIR command must be used with the
OPTION(SYNC) and MODE(RESYNC) parameters; see Table 18-1.
Table 18-1 CESTPAIR parameter values for the go-to-SYNC transition
Example 18-1 shows the TSO CESTPAIR command to change a Global Copy pair to
synchronous mode.
Operation Volume pair state CESTPAIR parameter values
from to OPTION MODE
go-to-SYNC
catch-up
duplex pending XD
duplex
SYNC RESYNC