RDF/IMP, IMPX, and ZLT System Management Manual

Introducing RDF

HP NonStop RDF/IMP, IMPX, and ZLT System Management Manual—524388-002

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Sorted Image Trails

RDF maintains its image data on disk volumes specified during RDF configuration. On

each of these volumes, the collection of files that contains image data is known as an

image trail; that is, there is one image trail per individual image trail volume.

The standard image trail used by RDF is called the master image trail. This image trail

is stored on the disk volume selected by the master receiver’s 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 master receiver’s

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.

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.