8.5.6. Journal Description

Red Hat Enterprise Messaging allows the size and number of files and caches used for persistence to be configured. There is one journal for each queue; it records each enqueue, dequeue, or transaction event, in order.

Each journal is implemented as a circular queue on disk, with a read cache and a write cache in memory. On disk, each circular queue consists of a set of files. The caches are page-oriented. When persistent messages are written to a durable queue, the associated events accumulate in the write cache until a page is filled or a timeout occurs, then the page is written to the circular queue using AIO. Messages in the write cache have not yet been acknowledged to the publisher, and can not be read by a consumer until they have been written to the journal. The page size affects performance - smaller page sizes reduce latency, larger page sizes increase throughput by reducing the number of write operations.

The journal files are prepared and formatted when the associated queue is first declared. This doubles throughput with AIO on the first pass, and also guarantees that needed space is allocated. However, this can result in a noticeable delay when durable queues are declared. When file size is increased, the delay is greater.