HP XP Racks - Manual (Page 10)

Failover software
Host failover software is used to transfer information between host servers at the local and remote
sites. It is a critical component of any disaster recovery solution.
•
When Continuous Access Synchronous is used as a disaster recovery tool, host failover is
required to ensure effective recovery operations.
•
When Continuous Access Synchronous is used as a data migration tool, host failover is
recommended.
Continuous Access Synchronous does not provide host failover functions. Use the failover software
most suitable for your platform and requirements (for example, Microsoft Cluster Server).
Overview of initial, update copy operations
When you perform the initial copy operation, all the data in the P-VOL is copied into the S-VOL.
Thereafter, the primary storage system runs update copy when it receives write instruction from the
host. The update operation copies host updates to the S-VOL at the same time that they are copied
to the P-VOL.
The following figure illustrates the initial copy and update copy operations.
Initial copy
When a new pair is created, the entire contents of the P-VOL are copied to the S-VOL cylinder by
track (this does not include diagnostic and unassigned alternate tracks). The initial copy synchronizes
the P-VOL and S-VOL independently of host I/O processes.
During the create pair operation, you can elect to have no data copied if data in the P-VOL and
S-VOL are already identical. You can also specify options that affect the speed of data transfer
and storage system performance. These are:
•
Copy Pace. You specify the number of tracks that can be copied simultaneously before the
primary system accepts another host request for that P-VOL. If more tracks are copied, the
operation completes quicker; if fewer tracks are copied, performance is maintained at a higher
level.
•
Maximum Initial Copy Activities. You specify the maximum number of concurrent initial copies
that the system performs.
•
Initial Copy Priority. You specify the order in which copying is performed on multiple pairs.
This is used if more pairs are being created in the operations than the maximum initial copy
activity setting.
•
Round Trip Time. You specify the time limit for data to travel from P-VOL to S-VOL. The value
is used by the storage system to control initial copy pace when update copying is in progress.
These options are available on Remote Web Console only.
Update copy
Update copy has a higher priority than initial copy. However, if an initial copy is in progress when
updates are sent by the host, the update copy must wait until the initial copy's copy pace completes.
For example, if the copy pace is 15 tracks, the update copy may wait up to 15 tracks (1 cylinder).
10 Continuous Access Synchronous overview