dbsizes.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: dbsizes.so,v 10.27 2005/02/24 20:51:50 bostic Exp $])

m4_ref_title(Access Methods,
    Database limits, database @limits, am_misc/stability, am_misc/diskspace)

m4_p([dnl
The largest database file that m4_db can handle depends on the page size
selected by the application.  m4_db stores database file page numbers as
unsigned 32-bit numbers and database file page sizes as unsigned 16-bit
numbers.  Using the maximum database page size of 65536, this results in
a maximum database file size of m4_sup(2, 48) (256 terabytes).  The
minimum database page size is 512 bytes, which results in a minimum
maximum database size of m4_sup(2, 41) (2 terabytes).])

m4_p([dnl
The largest database file m4_db can support is potentially further limited
if the host system does not have filesystem support for files larger than
m4_sup(2, 32), including the ability to seek to absolute offsets within
those files.])

m4_p([dnl
The largest key or data item that m4_db can support is m4_sup(2, 32),
or more likely limited by available memory.  Specifically, while key and
data byte strings may be of essentially unlimited length, any one of
them must fit into available memory so that it can be returned to the
application.  As some of the m4_db interfaces return both key and data
items to the application, those interfaces will require that any
key/data pair fit simultaneously into memory.  Further, as the access
methods may need to compare key and data items with other key and data
items, it may be a requirement that any two key or two data items fit
into available memory.  Finally, when writing applications supporting
transactions, it may be necessary to have an additional copy of any data
item in memory for logging purposes.])

m4_p([dnl
The maximum Btree depth is 255.])

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