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Since the beginning, Firebird had no rules of how to allocate disk space
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for database file(s). It just writes new allocated pages in not determined
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order (because of dependencies between pages to serve "careful write" strategy).
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This approach is very simple but has some drawbacks :
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- because of not determined order of writes, there may be such situation when
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page cache contains many dirty pages at time when new allocated page must be
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written but can't because out of disk space. In such cases often all other
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dirty pages are lost because administrators prefer to shutdown database
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before making some space on disk available. This leads to serious corruptions.
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- allocating disk space by relatively small chunks may lead to significant file
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fragmentation at file system level and reduce performance of large scans (for
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example during backup).
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Using new ODS 11.1, Firebird changes its disk space allocation algorithm to
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avoid corruptions in out of disk space conditions and to give the file system a
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chance to avoid fragmentation. These changes are described below.
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a) Every newly allocated page is written on disk immediately before returning to
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the engine. If page can't be written then allocation doesn't happen, PIP bit
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is not cleared and appropriate IO error is raised. This error can't lead to
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corruptions as we have a guarantee that all dirty pages in cache have disk
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space allocated and can be written safely.
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This change makes one additional write of every newly allocated page compared
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with old behavior. So performance penalty is expected during the database file
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growth. To reduce this penalty Firebird groups writes of newly allocated pages
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up to 128KB at a time and tracks number of "initialized" pages at PIP header.
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Note : newly allocated page will be written to disk twice only if this page
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is allocated first time. I.e. if page was allocated, freed and allocated again
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it will not be written twice on second allocation.
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b) To avoid file fragmentation, Firebird used appropriate file system's API to
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preallocate disk space by relatively large chunks. Currently such API exists
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only in Windows but it was recently added into Linux API and may be implemented
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in such popular file system's as ext2, etc in the future. So this feature is
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currently implemented only in Windows builds of Firebird and may be implemented
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in Linux builds in the future.
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For better control of disk space preallocation, new setting in Firebird.conf
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was introduced : DatabaseGrowthIncrement. This is upper bound of preallocation
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chunk size in bytes. Default value is 128MB. When engine needs more disk space
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it allocates 1/16th of already allocated space but no less than 128KB and no more
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than DatabaseGrowthIncrement value. If DatabaseGrowthIncrement is set to zero then
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preallocation is disabled. Space for database shadow files is not preallocated.
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Also preallocation is disabled if "No reserve" option is set for database.
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Note : preallocation also allows to avoid corruptions in out of disk space
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condition - in such case there is a big chance that database has enough space
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preallocated to operate until administrator makes some disk space available.
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Author: Vlad Khorsun, <hvlad at users sourceforge net>