Thanks for the question, Colm.
Asked: March 22, 2022 - 11:12 am UTC
Last updated: April 01, 2022 - 2:01 am UTC
Version: 11g
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Hi Tom,
Using Logminer as part of a Change Data Capture solution, the Logminer session is started with the continuous_mine option and a startscn parameter.
Each archived redo log is moved off disk to a backup location about 4 hours after archival on disk. This should be ok because the CDC product using Logminer should have already processed all transactions it was interested in from each log file by that time. However, we get errors complaining about the missing log file moved off disk, even when the CDC product has been up to date and idle for a number of hours.
My question is:
A. Is it ok that the Logminer session continues to run, even though the startscn that was specified when the session started is no longer accessible, since the redo log containing that SCN has moved off disk?
B. Is it possible that Logminer will still want access to the startscn, several hours after starting Logminer, even when transactions close to the current time are being queried in the Logminer contents by the CDC product?
Thanks,
Colm
and Connor said...
For both those questions, my response would be - why take the risk?
Logminer *might* run OK when the archives have been gone, conversely it *might* want to look back to older logs in some niche circumstance. I can't say for sure, but think of things like long running queries needing a really old snapshot, or perhaps a delayed block cleanout operation needing to resurrect some undo. Moreover, if Logminer does not need old archive logs in 11g, who is to say that it wont need them in some other way in 12c or 19c or beyond.
If I'm pinning a CDC solution on Logminer, it is just not worth the risk of doing that. - I'd be planning onkeeping my archives for a duration that spans longer than any exceptional circumstance I intend to encounter.
Either that, or I'd be looking at Goldengate, because the more complexity you build into a CDC solution, the closer you end up to just being a "Goldengate subset" anyway - just with no support
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