Performance Aspects of Shadowing
Performance is not noticeably affected by shadowing in multi-user systems, even though the shadowing system needs more machine resources because more files need to be updated.
Applications do not have to wait for the shadows to be updated because this is performed in the background. Actually all updates to the disk, even to master databanks, are performed by background processes (except for updates to TRANSDB and its shadows).
In single-user systems no background process is used. This means that an application has to wait for the shadows to be updated.
If the updating of shadows is delayed it will cause TRANSDB to grow. This can happen for several reasons:
- A shadow has been set offline and forgotten. If this happens transactions will be buffered until it is set online again.
To check if a shadow is offline use "list shadowing information" or the The Performance Report.
- A shadow is corrupt. Updates on the shadow results in an I/O error, and are buffered in TRANSDB.
When this happens the operator is notified by the system. To check it use the The Performance Report.
- There are too few shadowing processes to update the shadows, or they get too little machine resources.
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