Read-only transactions in SuperServer could not write Header and TIP pages
immediately after edit
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Key: CORE-5434
URL: http://tracker.firebirdsql.org/browse/CORE-5434
Project: Firebird Core
Issue Type: Improvement
Components: Engine
Reporter: Vlad Khorsun
Currently engine writes contents of Header and TIP pages to disk immediately
after any change (for ex. transaction start increments value of "Next
transaction" counter, commit changes transaction state etc).
I.e. Header and TIP pages are written to disk not waiting for commit (flush).
This was done by at least two major reasons:
a) safety
b) in CS (and SC) page locks for Header and TIP pages is released immediately
after use to avoid AST overhead
In the case of many light read-only transactions these writes makes almost 100%
of whole transaction time.
Since read-only transactions does not change data it is safe to update on disk
contents of Header and TIP pages
somewhat later, for example when write transaction started (or committed) or
when whole page cache is flushed
at database shutdown.
Note, reason (b) doesn't affect SuperServer.
So, improvement is that read-only transaction will not force write of
Header\TIP page content to disk immediately after change
(in SS mode, of course). In the case of many light read-only transactions it
gives significant performance gain.
PS to address this issue in CS mode there is more complex solution, probably it
will be implemented in v4 a bit later
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