Kbase P41910: What is a Checkpoint?
| Autor |
Progress Software Corporation - Progress |
| Acesso |
Público |
| Publicação |
6/10/2010 |
|
Status: Verified
GOAL:
What is a Checkpoint?
GOAL:
How to reduce checkpoint times?
GOAL:
What is a Before-image Cluster
FACT(s) (Environment):
All Supported Operating Systems
Progress/OpenEdge Product Family
FIX:
What is a Before-image Cluster?
The BI file is organized into clusters on disk. The database manager's before-image log is divided into fixed-size units of storage space called a Before-Image Cluster. The size is a multiple of 16 ranging from 16 to 262,128 (16K to 256MB) and is composed of bi blocks. The database engine reads and writes the blocks as one Before-Image Block (1, 2, 4, 8, 16 KB). The bicluster and blocksize can be changed when the before-image log is truncated. When the before-image log is expanded, it is expanded one cluster at a time. Clusters are reused when the data they contain is no longer needed for transaction rollback or crash recovery. Whenever a cluster is filled an asynchronous checkpoint is initiated.
What is a Checkpoint?
A Checkpoint is a process by which the in-memory and on-disk state of the database are reconciled. As transactions are executed, changes are made to copies of parts of the database (database blocks) that have been brought into volatile memory. The version that is on disk becomes progressively more obsolete. During a checkpoint, all memory-resident database changes are written to stable storage, making the volatile and stabile copies consistent.
As the database engine writes data to the BI file, these clusters fill up. When a bicluster fills, the database engine must ensure that all modified database buffer blocks referenced by notes in that cluster are written to disk. This is known as a checkpoint. Checkpointing additionally ensures that clusters can be reused when available and that the database can be recovered in a reasonable amount of time. By reusing clusters, the database engine minimizes the amount of disk space required for the BI file.
The BI cluster size combined with the application's write behavior a