HP 3PAR Remote Copy Software - Manual (Page 39)

2.23 Remote Copy Operation
InForm OS Version 2.3.1 Remote Copy User’s Guide
2.9.1.1 Synchronous Mode
In synchronous mode, a host-initiated write is performed on both the primary and the backup
storage servers before acknowledging the host write. On the primary storage server, data is
written to the caches of two nodes. This redundancy is in place in case one node fails before
the write can be copied to a disk. Concurrently, the write request is sent to the backup storage
server through a communication link. The backup storage server writes the same information
into its cache (again, on two nodes) and then sends an acknowledgement to the primary
system. The host write is acknowledged after the active cache update completes and the
backup acknowledgement is received ( Figure 2-13 ).
Figure 2-13. Synchronous Mode Remote Copy
Synchronous copying keeps the primary and backup storage servers synchronized at all times
and provides a higher level of data integrity compared to the asynchronous periodic mode
(explained in the following sections). No acknowledged I/O is lost even if the primary storage
server, the backup storage server, or the communication links go down. In the case of a disaster
affecting the primary storage server, the copy stored on the backup storage server at the
remote site is an exact replica and can be immediately used to continue the application.
Primary
Backup
Storage Server
Storage Server
Host
Write request
Write request forwarded
Data written to cache
on two nodes
Data written to cache
Backup acknowledges primary
1
2
3
4
5 6
Primary acknowledges host
on two nodes