HP XP P9500 Storage - Manual (Page 6)

1 Continuous Access Synchronous overview
Unless otherwise specified, the term P9000 in this guide refers to the following disk array:
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P9500 Disk Array
The GUI illustrations in this guide were created using a Windows computer with the Internet Explorer
browser. Actual windows may differ depending on the operating system and browser used. GUI
contents also vary with licensed program products, storage system models, and firmware versions.
Continuous Access Synchronous helps you create and maintain a synchronous backup of critical
data in a remote location. This manual provides information and instructions for planning,
configuring, creating, maintaining, monitoring, and troubleshooting a Continuous Access
Synchronous system on P9500 storage systems.
A Continuous Access Synchronous 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 P9500 system. This happens when data is written from the host to the
local system then to the remote system, via the Fibre Channel data path. The host holds subsequent
output until acknowledgement is received from the remote 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.
How Continuous Access Synchronous works
A pair is created when you:
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Select a volume on the local system that you want to copy.
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Create or identify the volume on the secondary system that will contain the copy.
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Connect the local and remote systems with a Fibre Channel data path.
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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), insuring that the secondary copy is a complete and consistent backup.
When a pair is split, writes to the primary volume continues but are no longer copied to the
secondary side, and the pair is no longer synchronous.
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If a special S-VOL write option is enabled, the secondary volume becomes available for read
or write access by secondary host applications. Otherwise the S-VOL remains as it was at the
time of the split.
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Changes to primary and secondary volumes (if applicable) are tracked by differential bitmaps
until the primary and secondary volumes are resynchronized.
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When resynchronization takes place, only the changed data is transferred, reducing copy
time.
The P9500 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 is used, the copy processing of Continuous Access
Synchronous is performed in addition to the processing to the volume not allocated to the Continuous
Access Synchronous pair. Therefore, there is a possibility that the performance decreases compared
with the case where Continuous Access Synchronous is not used and rises utilization rates.
To plan and implement a Continuous Access Synchronous system, an understanding of its
components is required. This is provided in the following sections.
6 Continuous Access Synchronous overview