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Berkeley DB Reference Guide: Access method FAQ
- Berkeley DB Reference Guide:
- Access Methods
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Access method FAQ
- I'm seeing database corruption when creating multiple databases
in a single physical file.
This problem is usually the result of DB handles not sharing an
underlying database environment. See Opening multiple databases in a single file for more information.
- Is there any way to compact databases, or return unused
database pages to the filesystem?
When Berkeley DB database pages are emptied, they are made available for other
uses, that is, new pages will not be allocated from the underlying
filesystem as long as there are unused pages available. However, the
pages cannot be returned to the filesystem without dumping the database,
removing the physical file, and reloading the database. The one
exception to this rule is Queue access method extent files. Queue
extent files removed when they are emptied, and their pages returned to
the underlying filesystem.
- I'm using integers as keys for a Btree database, and even
though the key/data pairs are entered in sorted order, the page-fill
factor is low.
This is usually the result of using integer keys on little-endian
architectures such as the x86. Berkeley DB sorts keys as byte strings, and
little-endian integers don't sort well when viewed as byte strings.
You may want to convert the keys to flat text or big-endian
representations, or provide your own Btree comparison function.
- Is there any way to avoid double buffering in the Berkeley DB system?
While you cannot avoid double buffering entirely, there are three
different tuning knobs you can work with to address this issue:
First, the Berkeley DB cache size can be explicitly set. Rather than allocate
additional space in the Berkeley DB cache to cover unexpectedly heavy load or
large table sizes, double buffering may suggest you size the cache to
function well under normal conditions, and then depend on the file
buffer cache to cover abnormal conditions. Obviously, this is a
trade-off, as Berkeley DB may not then perform as well as usual under abnormal
conditions.
Second, depending on the underlying operating system you're using, you
may be able to alter the amount of physical memory devoted to the file
buffer cache. Running as the system super-user makes a difference for
some UNIX or UNIX-like operating systems as well.
Third, changing the size of the Berkeley DB environment regions can change
the amount of space the operating system makes available for the file
buffer cache, and it's often worth considering exactly how the operating
system is dividing up its available memory. Further, moving the Berkeley DB
database environment regions from filesystem backed memory into system
memory (or heap memory), can often make additional system memory
available for the file buffer cache, especially on systems without a
unified buffer cache and VM system.
Copyright Sleepycat Software
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