Mongodb - nimrody/knowledgebase GitHub Wiki

Internals

  • A technical introduction to Wiredtiger

    • Can avoid fsync'ing the journal file after every write -- may lose some recent data.

    • Can avoid writing a journal file at all -- mongo will restore to the last snapshot

    • WT always writes updates in a different block. After the data and metadata had been fsync'ed -- old data is freed.

    • WT compresses on the way to disk. Keeps data uncompressed in memory (but indexes use additional prefix compression which affects memory too)

    • Pages are stored in memory as the original page + a skip list of changes. (may contain multiple documents)

  • Internals presentation

    • Probably out of date. Does not describe WiredTiger

  • Query all configuration parameters: db._adminCommand({getParameter:"*"})

  • Query all command line and config file parameters: db._adminCommand( {getCmdLineOpts: 1})

  • On multi-tenancy

  • Sharding and other deployment advice

Useful commands

  • backup database mongodump --host nir-lnx-10 --out /mnt/c/home/apm --gzip (or add --db=apm for specific database only)

  • restore database mongorestore /mnt/c/home/apm --gzip (or add --db=apm for specific database)

  • Change TTL of index (see working with mongodb TTL)

    use apm db.runCommand({collMod : 'records', index: {name: 'created_1', expireAfterSeconds:(100*86400)}})

  • Get configuration db._adminCommand({getParameter:"*"})

  • Aggergation framework presentation

PyMongo

  • Get the collection stats

    m=MongoClient('localhost') m['apm'].command('collstats', 'flat_apm2')

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