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94 Storage Management with DB2 for OS/390
one-to-one redundancy. Its purpose is I/O automatic switching to secondary when
unattended outage occurs on primary.
Virtual volume, RAID 5, and RAID 6 have made the concept of dual copy
practically obsolete
.
9.5.2 Concurrent Copy
Concurrent copy is a function the disk storage server controls in conjunction with
DFSMS software. Concurrent copy enables taking backup copies of data while
minimally impacting the application data access. Concurrent copy delivers a
consistent point-in-time copy of a table space while allowing minimum
interruption to data access. Table spaces remain available for processing during
almost the entire backup process. They must be switched into read mode for only
a short time during concurrent copy initialization.
Initiating a concurrent copy operation allows DFSMSdss to build a map of the
data to be copied. To create this map in the case of update I/Os, DFSMSdss
needs to obtain serialization on the data while the application I/O is suspended.
This process establishes the concurrent copy session (or logical copy); then
application access to the data can be resumed, and the copy is available for
DFSMSdss DUMP or COPY functions.
With concurrent copy, consistent copies of DB2 data objects can be taken almost
instantaneously, thus significantly reducing the length of system outage required
for backup. The elapsed time for a backup is reduced from the time taken to
physically back up the data to a minimum time to establish the concurrent copy
session. Either the database must be quiesced, or the table locked in read-only
access for the data to be serialized. More backups can be captured and therefore
reduce the length of time taken for forward recovery in the event of a recovery.
Figure 26 on page 94 shows the difference between a traditional backup and a
concurrent copy backup.
Figure 26. Schema of a Backup with Concurrent Copy
APPLICATION PROCESSING
APPLICATION PROCESSING
Backup Window
With Concurrent Copy
Without Concurrent Copy
APPLICATION PROCESSING
APPLICATION PROCESSING