For a DASD-only log stream data is duplexed to DASD staging data
sets as it is written to the log stream. This means that the chance
of losing data because of a system failure is far less; There is no
lag time between a successful write and hardening of the data on DASD.
And of course, a coupling facility failure will not affect a DASD-only
log stream. However, it can be helpful for an application to use the
IXGQUERY service to ensure that log data is safe to import from a
DASD-only source
log stream. For a
DASD-only log stream, the safe import point is the
highest log block identifier that has been successfully written to
the log stream. The safe import point for a
DASD-only is updated with
each successful write, because the log data is simultaneously duplexed
to DASD staging data sets. Any log blocks in the source log stream
with a block identifier less than or equal to the safe import point
can be safely imported to the target log stream. An application can
determine the safe import point by doing one of the following:
- Issue IXGQUERY, which returns the safe import point.
- Listen for ENF event 48 indicating that the safe import point
has been updated, (field IXGENFWrOffLoadSafeImportPoint in the parameter
list mapped by macro IXGENF contains the latest safe import point.)
The ENF is issued when an offload operation completes. Notification
that the offload has completed is presented on all systems in the
sysplex that have a connection to the log stream.