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Файл:База данных / IS_to_STUD / Soft / 2.1.0.17798-Final / Firebird-2.1.0.17798-0_Win32 / doc / README.garbage_collector
.txt -----------------------------------------------------------
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
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
Соседние файлы в папке doc