62 IMS Version 8 Implementation Guide
During a batch run when the application program issues either a CHKP or a ROLB call, IMS
will determine whether it should coordinate the subsequent syncpoint itself or participate in
the syncpoint coordinated by RRS.
It determines this by expressing interest in the current UOW, and then retrieving the count of
interests expressed. If the count is one, then the lone interest must be IMS's itself and it then
deletes its own interest and coordinates the syncpoint itself.
However, if there has been more than one interest expressed, then batch will initiate the RRS
syncpoint via the ATRCMIT (or ATRBACK) call and assume the role of a syncpoint participant
to RRS.
4.5 Coordinated IMS/DB2 disaster recovery support
There have been additional enhancements to aid in the coordination of disaster recovery for
installations accessing DB2 and IMS. They are in response to user requirements asking for
synchronized disaster recovery support using independent transfer mechanisms to send both
IMS and DB2 logs to the remote site. You will find further information on Coordinated
IMS/DB2 disaster recovery support in
IMS Version 8: Release Planning Guide,
GC27-1305.
The enhancements are based on the usage of Remote Site Recovery (RSR) for IMS and
eXtended Remote Copy (XRC) for your DB2 environment. RSR utilizes APPC to send IMS
log data from the active site to the remote site. The RSR tracker instance at the remote site
which is writing its copy of log records received from the active site is also recording into the
tracking RECON data set and optionally updating its own shadowed databases if running on
database level tracking (DLT), shown in Figure 4-1,
Figure 4-1 IMS RSR
XRC is a function of DASD storage servers (3990 control units and Enterprise Storage Server
(ESS)) and is using DFSMS System Data Mover (SDM) for asynchronous mirroring of the
necessary DB2 logs and boot strap data sets as shown in Figure 4-2.
IMS Databases Shadow IMS Databases
(optional)
IMS Log Records
APPC
IMS
RSR Tracker
IMS
Active Site
IMS Logs
Chapter 4. Database management enhancements 63
Figure 4-2 DB2 XRC
For more information about XRC please refer to IBM Redbook
Implementing ESS Copy
Services on S/390
, SG24-5680, Chapter 3, XRC.
Coordinated recovery operations now allow users to recover IMS and DB2 data to a
consistent point in time. The highlights are:
򐂰 eXtended Remote Copy (XRC) tracking is added to IMS RSR
򐂰 IMS and DB2 log synchronization for disaster recovery
򐂰 Support of RSR for IMS logs and optionally, shadowed databases
򐂰 Support of XRC for DB2 logs, together with the boot strap data sets (BSDS) on a single
XRC session to keep both synchronized to each other; DB2 databases (tables) may not
be included
This support is especially interesting for environments with limited bandwidth for XRC
transmissions. Those environments probably cannot support the transmission of the entire
DB2 databases and their updates. Instead, only the logged DB2 and IMS data and the DB2
BSDSs are transmitted.
IMS TM Version 8 can be connected to the DB2 Version 6 and Version 7. The DB2 logs and
BSDSs must reside on devices supporting XRC. DB2 must be running in data sharing mode
since this mode provides timestamps in the DB2 log. Without data sharing mode the DB2
subsystem would use RBAs instead.
4.5.1 XRC tracking
Since XRC tracking is implemented in our IMS RSR process the IMS truncation point is
issued in the new
DFS2933I
message.
The IMS log truncation point will be kept behind XRC consistency time. This means the RSR
tracking subsystem (the log router function) ensures that the routing of IMS logs is always
behind that of the DB2 logs. To get this information, the RSR tracker will frequently invoke an
XRC Query API request against the DFSMS system data mover (SDM). This XRC tracking
is running under its own ITASK and is using SDMs API provided by the ANTRQST macro call
REQUEST(XCONTIME). So it is a requirement that the SDM is running on the same OS/390
system as the IMS RSR tracking subsystem is running. Otherwise the previously invoked API
Storage Server Storage Server
DFSMS
System Data Mover
ESCON with
DASD Channel Extenders

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