HP 3PAR Remote Copy Software - Manual (Page 41)

2.25 Remote Copy Operation
InForm OS Version 2.3.1 Remote Copy User’s Guide
2.9.2 Synchronization Types
There are two types of synchronizations that are performed on volume groups: full
synchronizations and resynchronizations.
A full synchronization copies the primary volume in its entirety, whereas a resynchronization
copies only what has changed after the previously completed synchronization. A new volume
group always requires a full synchronization at first, whether it is an asynchronous periodic
volume group or a synchronous volume group (see Volume Group Modes on page 2.22 to
understand the difference).
When a synchronization is initiated, with the exception of the initial synchronization,
snapshots are created on the backup storage server. This stage is required to ensure that the
backup system maintains a valid point-in-time copy at all times. The system must always
maintain a valid point-in-time copy because a failed synchronization attempt could leave the
base volumes in an inconsistent state.
2.9.2.1 Asynchronous Periodic Mode Volume Groups
For the asynchronous periodic mode volume groups, after the snapshots are created on the
backup storage server, all volumes in the primary volume group (on the primary storage server)
accept new writes, but hold the acknowledgement. While new writes are held, the primary
system creates snapshots of all volumes within the group. After creating these snapshots, the
system allows I/O to resume and acknowledges any writes that were held ( Figure 2-15 ).
NOTE: Any writes sent from the host are accepted, but not committed or
acknowledged until after the snapshots are completed. This is to ensure that the
host will not see any I/O failure.