source-oracle: Per-thread log file selection for RAC#4490
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In Oracle RAC, each instance writes its own thread of redo, and SCN ranges across threads overlap arbitrarily while sequence numbers are independent. The previous log selection logic assumed a single thread, and could misbehave in all kinds of interesting ways when that wasn't true. The new selection logic sorts candidate files by `FIRST_CHANGE#` and tracks `NEXT_CHANGE#` per-thread then takes the minimum across threads to find the global covered SCN upper-bound after adding each file. This logic reduces to the old behavior in single-threaded cases. I have not tested this against a real RAC deployment but I believe that it should work in theory, and as previously mentioned it reduces to the single-thread logic the rest of the time so I believe it should be safe to try in production.
willdonnelly
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Description:
In Oracle RAC, each instance writes its own thread of redo, and SCN ranges across threads overlap arbitrarily while sequence numbers are independent. The previous log selection logic assumed a single thread, and could misbehave in all kinds of interesting ways when that wasn't true.
The new selection logic sorts candidate files by
FIRST_CHANGE#and tracksNEXT_CHANGE#per-thread then takes the minimum across threads to find the global covered SCN upper-bound after adding each file. This logic reduces to the old behavior in single-threaded cases.I have not tested this against a real RAC deployment but I believe that it should work in theory, and as previously mentioned it reduces to the single-thread logic the rest of the time so I believe it should be safe to try in production.