Firebird 2.0 garbage collector
Engine knows about each record
versions produced by update\delete statement and can remove them as
soon as oldest snapshot transaction (OST) allowed this. This
eliminates the need to read pages with these versions again by user
request (i.e. SELECT COUNT(*) FROM table) and avoids situation when
these versions are never read (of course sweep always removed all
unused record versions). Also there is high probability that needed
pages still reside in the buffer cache thus there are less additional
disk IO needed.
Between notifying garbage collector
about page with unused versions and time when garbage collector will
read this page new transaction can update record and garbage
collector can't cleanup this record if this new transaction number is
above OST or still active. In this case engine again notifies garbage
collector with this page number and it will clean up garbage at some
time later (old garbage collector will "forget" about this
page until user reads it again).
Both cooperative and background
garbage collection are now possible. To manage it new configuration
parameter "GCPolicy" was introduced. It can be set to:
a) cooperative - garbage
collection performed only in cooperative mode, such as before IB6.
Each user request is responsible for removing unused record
versions. This is how engine works before IB6. This is only option
for Classic Server mode. Engine doesn't track versions produced as
result of update and delete statements
b) background - garbage
collection performed only by background thread, such as in IB6 and
later. No user requests remove unused record versions. Instead user
request notifies dedicated garbage collector thread with page number
where unused record version is detected. Also engine remembers page
numbers where update\delete statement created backversions.
c) combined - both
background and cooperative garbage collection are performed.
Default is "combined" for
SuperServer. ClassicServer ignores this parameter and always works in
"cooperative" mode
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