HP XP P9500 Storage - Manual (Page 7)

1 Continuous Access Synchronous overview
A Continuous Access Synchronous (Cnt Ac-S) system creates and maintains a mirror image of a
production volume at a remote location. Data in a Continuous Access Synchronous backup stays
synchronized with the data in the local XP P9500 storage system. This happens when data is
written from the host to the local storage system then to the remote storage system, via the Fibre
Channel data path. The host holds subsequent output until acknowledgement is received from the
remote storage system for the previous output.
Continuous Access Synchronous can be teamed with Business Copy or Continuous Access Journal,
on either or both local and remote sites. These copy tools allow restoration from one or more
additional copies of critical data.
This guide provides instructions for planning, configuring, operating, maintaining, and
troubleshooting a Continuous Access Synchronous system.
How Continuous Access Synchronous works
A pair is created when you:
•
Select a volume on the local storage system that you want to copy.
•
Create or identify the volume on the secondary system that will contain the copy.
•
Connect the local and remote storage systems with a Fibre Channel channel data path.
•
Copy all primary volume data to the secondary volume.
During a typical initial copy, all data written to the primary volume (P-VOL) is copied to the
secondary volume (S-VOL), ensuring that the secondary copy is a complete and consistent backup.
When a pair is split, writes to the primary volume continue but are no longer copied to the secondary
side, and the pair is no longer synchronous.
•
If a special S-VOL write option is enabled, the secondary volume becomes available for
read/write access by secondary host applications. Otherwise the S-VOL remains as it was at
the time of the split.
•
Changes to primary and secondary volumes (if applicable) are tracked by differential bitmaps
until the primary and secondary volumes are re-synchronized.
•
When resynchronization takes place, only the changed data is transferred, reducing copy
time.
The XP P9500 storage system transfers control parameters and FBA-format data for consecutive,
updated records in a track using a single write operation. This eliminates the overhead that is
usually required for performing FBA-to-CKD and CKD-to-FBA conversions.
When Continuous Access Synchronous operations and other storage system processing are
performed at the same time, performance is affected.
To plan and implement a Continuous Access Synchronous system an understanding of its components
is required. This is provided in the following sections.
Typical components
A typical configuration consists of the following components. Many but not all require user setup.
•
The local P9500 storage system is connected to a host. The remote storage system is connected
to the local system via Fibre Channel data paths. The secondary system may be may be a
P9500 or a different model, such as an XP24000/XP20000 Disk Array, an XP12000 Disk
Array/XP10000 Disk Array, HUS VM or a vendor storage systems.
•
A host at the local site, connected to the local storage system. It is also highly desirable to
have a host at the secondary site connected to the secondary system for use in a disaster
How Continuous Access Synchronous works 7