本文适合于对mongodb有一定了解的朋友阅读。 mongodb的数据文件存在dbpath选项指定的目录里。每个库(database)都有一系列的文件：dbname.ns, dbname.0, dbname.1, ...数据文件也叫pdfile，意思是Portable Data File。 dbname.ns文件 dbname.ns文件存储命名空
本文适合于对mongodb有一定了解的朋友阅读。
mongodb的数据文件存在dbpath选项指定的目录里。每个库(database)都有一系列的文件：dbname.ns, dbname.0, dbname.1, ...数据文件也叫pdfile，意思是Portable Data File。
dbname.ns文件dbname.ns文件存储命名空间信息。在mongodb里，每个collection都具有一个命名空间，名字为dbname.collection_name。dbname.ns文件存储的是一个哈希表节点数组。key是根据命名空间的名字，value是命名空间信息。哈希表节点的大小是628字节，香港服务器租用，dbname.ns文件的默认大小是16M，香港虚拟主机，一共可以存放26715个命名空间。nssize选项可以设置dbname.ns文件的大小。
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dbname.系列文件dbname.系列文件存储了每个库的所有数据，其文件格式为
--------------------------------------------
DataFileHeader
--------------------------------------------
Extent (for a particular namespace)
Record
...
Record (some chained for unused space)
--------------------------------------------
more Extents...
--------------------------------------------
DataFileHeader是数据文件的头部，后面的部分为Extent。文件空间的分配以Extent为单位。每个命名空间的所申请的Extent形成一个双向链表，表头和表尾存在命名空间信息里。Record即记录，在Extent里分配，每个Extent里的所有Record形成一个双向链表，表头和表尾存在Extent头部。可以想到，对命名空间的所有Record的遍历方法为：遍历Extent链表，对每个Extent，遍历其Record链表。空闲的Record(Extent里剩余的空间、或者Record被删除)，免备案空间，称作DeleteRecord，根据其大小，形成19个单向链表（表头也存在命名空间里）。可以想到，申请一个Record的方法：先从空闲的Record里面找；如果找不到，则分配新的Extent。
当一个命名空间被删除的时候，它的所有的Extent都会挂到名为$freelist的collection的Extent链表中。那么，分配Extent的时候，会先从$freelist的Extent链表中寻找。如果找不到，就申请新的Extent。
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