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MongoDB - Disk space management

  • The tests were performed in MongoDB v3.2.9.
  • It does not matter what engine you're using in your MongoDB instance, the space released of a removed document is not available to the system, unless you execute specific commands, such compact (collection level) or repairDatabase (database level);
  • A collection must exist in order to execute the command compact;

Each engine have unique behaviors, I'll list what I've observed below.

MMAPv1

  • The space is only released to the system if the command repairDatabase is executed;
  • The space released of a dropped collection is not available to the system;
  • The space released from the command compact is not released to the system;

WiredTiger

  • The space released of a dropped collection is a available to the system;
  • The command compact releases spaces to the system, just like repairDatabase;

Code for tests

// g++ main.cpp -g3 -std=c++14 -I/usr/include/mongocxx/v_noabi -I/usr/include/bsoncxx/v_noabi -I/usr/include/libbson-1.0 -lbsoncxx -lmongocxx
#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
#include <mongocxx/uri.hpp>

#include <bsoncxx/builder/basic/document.hpp>
#include <bsoncxx/builder/basic/kvp.hpp>
#include <iostream>

/**
 * this code was written to test how mongo deals with disk space release
 */
int main (int argc, char** argv)
{
  if (argc < 3)
  {
    std::cout << "[bin] [dbName] [collName]" << std::endl;
    return 1;
  }

  try
  {
    std::string dbName(argv[1]);
    std::string collName(argv[2]);

    mongocxx::instance instance{};
    mongocxx::client conn{ mongocxx::uri{} };

    auto db = conn[dbName];

    // remove all
    auto removeFilter = bsoncxx::builder::basic::document{};
    db[collName].delete_many(removeFilter.view());

    // compact coll after remove
    auto compactComm = bsoncxx::builder::basic::document{};
    compactComm.append(bsoncxx::builder::basic::kvp("compact", collName));
    db.run_command(compactComm.view());

    // drop collection
    db[collName].drop();

    // repair database to release space
    auto repairDbComm = bsoncxx::builder::basic::document{};
    repairDbComm.append(bsoncxx::builder::basic::kvp("repairDatabase", 1));
    db.run_command(repairDbComm.view());
  }
  catch (const std::exception& e)
  {
    std::cout << e.what() << std::endl;
  }
}

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