RDF/IMP and IMPX System Management Manual (RDF 1.3+)

Introducing RDF
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
1-20
Purger Process
Reading Image Data
Write operations to the various sorted image trails occur asynchronously to one another.
To ensure correct operation, the updaters cannot read to the end-of-file. Instead, they can
only read as far as the receiver allows (determined by receiver “save” points in the image
trail). Thus, on a finely tuned RDF backup node, the RDF time delay (RTD) for an
updater can regularly lag 5 to 50 seconds behind TMF processing. This 50-second delay
does not mean that 50 seconds are needed for the updater to catch up, however; that
operation will typically require only a few seconds.
Purger Process
With RDF/IMP and IMPX, the updaters apply all audit records to their data volumes
regardless of whether the associated transaction has committed, has aborted, or is still in
progress.
The purging of redundant image trail files is based on transaction information.
Specifically, the receiver process maintains general information on what transactions
might be in each image file. This information is system-wide, not specific to any
particular image trail. The reasons for this pertain to performance.
First, if the receiver had to maintain specific information about what transactions were
actually represented in each image file on each image trail, the extractor-receiver
performance rate would be seriously degraded. Therefore, the receiver keeps general
information about all transactions it has seen across all trails.
Second, because considerable checking must be done across all trails to determine what
files can be purged based on what transactions might be represented in the various files
on the various image trails, the purger process performs this task.
The purger process is a restartable process pair that runs on the backup system (it is
started during START RDF and runs even when the updaters are stopped; image files are
purged, however, only when updating is enabled).
No image file in a given image trail can be purged until it is absolutely clear that all
updaters configured to the trail will no longer require that file for an UNDO pass. RDF
automatically keeps track of which range of transactions is represented in each image
trail file. The purger process can therefore always determine with confidence when a
particular image trail file can be purged.
For example, assume the following:
•
There are two image trails.
•
Five updaters are assigned to each trail.
•
A long-running transaction (T1000) involves all five updaters on one trail, but none
on the other.
•
T1000 became active when the current image file in each trail was AA000002, and
is still active.