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

Introducing RDF
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
1-16
Image Trail Buffers and Files
The standard image trail used by RDF is called the master image trail. This image trail is
stored on the disk volume selected through the RDFVOLUME configuration option.
Note that you cannot configure any updaters to the master image trail.
All updaters must be configured to secondary image trails. You can configure up to 255
secondary image trails in addition to the master image trail. Each secondary image trail
is stored on a separate volume, selected through the IMAGETRAIL configuration
option.
RDF uses multiple sorted image trails. With this feature, the receiver detects which
updaters are associated with which image trails. When it receives a record, the receiver
identifies the updater that will apply the record to the backup database. The receiver then
sorts the record into the appropriate image trail, and the record is written to that image
trail.
With sorted image trails, only one image trail contains the transaction status records that
hold key information about whether a transaction has committed or aborted; this trail is
the master image trail, stored on the volume selected by the RDFVOLUME
configuration option.
With sorted image trails, the activity of any one image file typically remains so low that
it can be stored on the same disk volumes as the main database with no significant I/O
impact. This approach is not recommended, however, if you require very high RDF
performance or if RDF is running with the UPDATE option turned off; in this case, the
image trails could eventually fill the volume; in such cases, it is best to have volumes
exclusively dedicated to the image trails.
Image trails can be added only after RDF has been initialized but before it has been
started.
Image Trail Buffers and Files
At the backup node, the receiver maintains two buffers for each image trail. The receiver
then sorts the image records into the appropriate buffers. When a buffer fills, it is written
to its respective image file, and the receiver begins filling the second buffer. In normal
operation, the receiver never waits for image file I/O to complete, because the I/O time is
shorter than the time needed to fill the next buffer.
Updater Processes
An updater process is a process pair that runs on the backup system when updating is
enabled or during takeover processing. Every volume on the primary system that is
protected by RDF has its own updater process on the backup system.
Each updater reads the image trail to which it has been configured, looking for audit
information associated with the data volume it protects (it ignores audit information
associated with volumes protected by other updaters). When it finds applicable audit
Note. You should keep all image trail files off of the $SYSTEM volume and its controller.
Otherwise, if there is a lot of audit data to send from the primary system to the backup system,
it could take a while for the updaters to start.