HP StorageWorks XP24000 Continuous Access Software User and Reference Guide, v01 (T5278-96002, June 2007)

Data(V)
Primary volume
Log(V)
Data(V)
Secondary volume
Log(IV)
Figure 53 Relationship B etween Log File and Data File in PAIR Status
Since most UNIX file systems (excluding JFS and VxFS) have no journal files, theP-VOL fencelevel should
be
defined as Never. When a takeover by the S-VOL occurs, fsck is executed on the volume a nd the
file system is
cleaned up, even if the S-VOL is u ndefined at the secondary host. The data that will be
lost depends on how much differential data is contained in the P-VOL when the S-VOL is suspended.
During operation, error recovery should be performed when the suspended status (PSUE) is detected
(when one er
ror occurs).
Transferring Sense Information Between Sites
When the MCU (or RCU for Continuous Access Asynchronous) suspends a Continuous Access pair due to
an
error condition, the MCU/RCU sends sense information with the unit check status to the appropriate
host(s). This sense information is used during disaster recovery to determine the currency of the S-VOL,
andmustbe transferred to theremotesitevia the hostfailoversoftware.
File and Database Recovery Procedures
When a Continuous Access Synchronous pair is suspended, or when the MCU fails due to a disaster, the
S-VOL may contain in-process data. A data set could b e open, or transactions may not have completed.
Even if yo
uuse the Data fence level for all Continuous Access Synchronous pairs, you need to establish
file recovery procedures. These procedures should be the same as those used for recovering any volume
which becomes inaccessible due to control unit failure. These p rocedures are more impor tant if the Status
or
Never fencelevel settings areused.
Continuous Access Asynchronous does not provide any procedure for detecting and retrieving lost
updates. To detect and recrea te lost updates, you must check other current information (for example,
databa
se
journal log file that was active at the primary system when the disaster occurred). The
Contin
uous Access Asynchronous consistency status (group or volume) can be useful when performing
this
d
etection and retrieval. Since this detection/retrieval process can take a while, your disaster recovery
scenario should be designed so that detection/retrieval of lost updates is per formed after the application
has
been started at the secondary system.
You sh
ould prepare for file and database recovery by using:
• Files for file recovery (for example, database log files which have been verified as current). To
ensure the currency of these files, use the Data fence level setting for the Continuous Access pairs
which contain these important files.
• Thesense informationwillbetransferred viahostfailover.
Impo
rtant:
Remote copy and disaster recovery procedures are inherently complex. Consult your HP
service representative on sense-level settings and recovery procedures.
Note: See “PinnedTrack Recovery forContinuousAccessVolumes” on page 125 for information on
recovering a pinned track on a Continuous Access volume.
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