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MongoDB logical structure

It consists of three parts: document, collection and database.

MongoDB document: The basic unit of data in mongodb, equivalent to a row of records in a relational database. Each document has a special key "_id", which is unique in the collection to which the document belongs.

Collection: A concept equivalent to tables in relational databases.

The hierarchical structure of document, collection and database is as follows:

Document corresponds to row in relational data

A collection corresponds to a table in relational data.

database corresponds to relational database

The MongoDB data directory stores all MongoDB data files

[mongo@xxxx-xx-xx data]$ ll Total Usage 1212436drwxr-xr-x. 2 mongo dbmon 69 Oct 28 17:15 diagnostic.datadrwxr-xr-x. 2 mongo dbmon 27 Oct 28 15:25 journal-rw-----. 1 mongo dbmon 67108864 Oct 28 15:25 local.0-rw-----. 1 mongo dbmon 1073741824 Oct 28 17:15 local.1-rw-----. 1 mongo dbmon 16777216 Oct 28 17:14 local.ns-rw-r--r--. 1 mongo dbmon 6 Oct 28 15:17 mongod.lock-rw-r--r--. 1 mongo dbmon 69 Oct 28 15:17 storage.bson-rw-----. 1 mongo dbmon 67108864 Oct 28 17:00 sykdb.0-rw-----. 1 mongo dbmon 16777216 October 28 17:00 sykdb.nsdrwxr-xr-x. 2 mongo dbmon 6 Oct 28 15:54 _tmp

Inside MongoDB, each database contains a.ns file and a number of data files, such as local.ns,local.0,local.1, the number of files increasing as the data volume increases.

Each table has a namespace, and each index has a namespace. Metadata for these namespaces is stored in *.ns files.

MongoDB has an internal mechanism for pre-allocating space, pre-allocated files are filled with zeros, so mongodb always keeps extra space and spare data files. Each time a new data file is generated, it is twice the size of the previous data file. For example, local.0 is 67108864, local.1 is 1073741824

Reference: mongodb family-architecture

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