Backup and Restore in MongoDB

Backup and Restore

• backups generally should be done on secondaries
• or on standalone servers at an off time

Method #1: Filesystem Snapshot

• the simplest way to make backup
• filesystem must support snapshotting
• mongod must run with journaling enabled
• stop mongod before restoring

# Create the snapshot volume
lvcreate -L128M -s -n dbbackup /dev/ops/databases

# Mount the snapshot volume
mkdir /mnt/ops/dbbackup
mount /dev/ops/dbbackup /mnt/ops/dbbackup

# Do the backup
tar -cf /dev/rmt0 /mnt/ops/dbbackup

# Remove the snapshot
umount /mnt/ops/dbbackup
lvremove /dev/ops/dbbackup

Method #2: Copying Data Files

• use fsyncLock() before copying files
  • fsycnLock() prevents all databases against any further write
  • saves all dirty data to disk
  • queues all write operations
• now files are consistent and copying of all files is possible
• fsyncUnlock() releases the lock and brings back database to normal operations
• if using authentication do not log out from the console between fsyncLock() and fsyncUnlock()
• stop mongod before restoring
  • do not restore single database if crash or hard shutdown occurs

> db.fsyncLock()
> db.fsyncUnlock()

Method #3: Using mongodump and mongorestore

• mongorestore is slower than previous methods and has other downsides
• good way to backup individual databases or collections
• it will create a dump directory (with subdirectories for each database) in current directory
• data is stored in .bson files
• mongodump can be used when mongod is not running
  • do not use --dbpath when mongod is running
• when mongodump is running writes are allowed so already backuped data may change before mongodump will finish
  • do not use fsyncLock() when using mongodump
  • use --oplog if you are running mongod with --replSet
• mongodump will choose secondary when connected to replica set
• use mongodump and mongorestore in the same version
• try to avoid this type of backup if you have unique indexes other than _id

mongodump --help
mongodump --port 27017
mongodump --dbpath /var/lib/mongodb
mongorestore --port 27017 --drop
mongodump --port 27017 --oplog
mongorestore --port 27017 --oplogReplay dump/
mongorestore -d dstDB -c dstCollection dump/srcDB/scrColl.bson

Backup of Replica Set

• all previous methods are OK but 1st and 2nd are recommended (without any modifications)
• when using mongodump use --oplog
• when restoring with mongorestore:
  1. start a server as a standalone and
  2. restore the data with --oplogReplay and
  3. restore the oplog collection
  4. restart the server as a member of replica

> use local
> db.createCollection("oplog.rs", {"capped" : true, "size" : 1000000})
$ mongorestore -d local -c oplog.rs dump/oplog.bson

Backup of Sharded Cluster

• usually it can not be done perfectly
• instead of backing up everything at once, backup servers separately
• turn off balancer before making backup
• run mongodump through mongos to backup entire cluster
• problems with restoring single shard