extentsize.so

berkeley db 4.6.21的源码。berkeley db是一个简单的数据库管理系统

m4_comment([$Id: extentsize.so,v 1.3 2001/05/02 15:53:22 bostic Exp $])

m4_ref_title(Access Methods,
    Selecting a Queue extent size,
    selecting a Queue @extent size,
    am_conf/recno, am_conf/re_source)

m4_p([dnl
In Queue databases, records are allocated sequentially and directly
mapped to an offset within the file storage for the database.  As
records are deleted from the Queue, pages will become empty and will
not be reused in normal queue operations.  To facilitate the reclamation
of disk space a Queue may be partitioned into extents.  Each extent is
kept in a separate physical file.])

m4_p([dnl
Extent files are automatically created as needed and marked for deletion
when the head of the queue moves off the extent.  The extent will not
be deleted until all processes close the extent.  In addition, m4_db
caches a small number of extents that have been recently used; this may
delay when an extent will be deleted. The number of extents left open
depends on queue activity.])

m4_p([dnl
The extent size specifies the number of pages that make up each extent.
By default, if no extent size is specified, the Queue resides in a
single file and disk space is not reclaimed.  In choosing an extent size
there is a tradeoff between the amount of disk space used and the
overhead of creating and deleting files.  If the extent size is too
small, the system will pay a performance penalty, creating and deleting
files frequently.  In addition, if the active part of the queue spans
many files, all those files will need to be open at the same time,
consuming system and process file resources.])

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