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Backup and Restore with Filesystem Snapshots

This document describes a procedure for creating backups of MongoDB systems using system-level tools, such as LVM or storage appliance, as well as the corresponding restoration strategies.

These filesystem snapshots, or “block-level” backup methods use system level tools to create copies of the device that holds MongoDB’s data files. These methods complete quickly and work reliably, but require more system configuration outside of MongoDB.

Snapshots Overview

Snapshots work by creating pointers between the live data and a special snapshot volume. These pointers are theoretically equivalent to “hard links.” As the working data diverges from the snapshot, the snapshot process uses a copy-on-write strategy. As a result the snapshot only stores modified data.

After making the snapshot, you mount the snapshot image on your file system and copy data from the snapshot. The resulting backup contains a full copy of all data.

Snapshots have the following limitations:

  • The database must be valid when the snapshot takes place. This means that all writes accepted by the database need to be fully written to disk: either to the journal or to data files.

    If all writes are not on disk when the backup occurs, the backup will not reflect these changes. If writes are in progress when the backup occurs, the data files will reflect an inconsistent state. With journaling all data-file states resulting from in-progress writes are recoverable; without journaling you must flush all pending writes to disk before running the backup operation and must ensure that no writes occur during the entire backup procedure.

    If you do use journaling, the journal must reside on the same volume as the data.

  • Snapshots create an image of an entire disk image. Unless you need to back up your entire system, consider isolating your MongoDB data files, journal (if applicable), and configuration on one logical disk that doesn’t contain any other data.

    Alternately, store all MongoDB data files on a dedicated device so that you can make backups without duplicating extraneous data.

  • Ensure that you copy data from snapshots and onto other systems to ensure that data is safe from site failures.

  • Although different snapshots methods provide different capability, the LVM method outlined below does not provide any capacity for capturing incremental backups.

Snapshots With Journaling

If your mongod instance has journaling enabled, then you can use any kind of file system or volume/block level snapshot tool to create backups.

If you manage your own infrastructure on a Linux-based system, configure your system with LVM to provide your disk packages and provide snapshot capability. You can also use LVM-based setups within a cloud/virtualized environment.

Note

Running LVM provides additional flexibility and enables the possibility of using snapshots to back up MongoDB.

Snapshots with Amazon EBS in a RAID 10 Configuration

If your deployment depends on Amazon’s Elastic Block Storage (EBS) with RAID configured within your instance, it is impossible to get a consistent state across all disks using the platform’s snapshot tool. As an alternative, you can do one of the following:

Backup and Restore Using LVM on a Linux System

This section provides an overview of a simple backup process using LVM on a Linux system. While the tools, commands, and paths may be (slightly) different on your system the following steps provide a high level overview of the backup operation.

Note

Only use the following procedure as a guideline for a backup system and infrastructure. Production backup systems must consider a number of application specific requirements and factors unique to specific environments.

Create a Snapshot

To create a snapshot with LVM, issue a command as root in the following format:

lvcreate --size 100M --snapshot --name mdb-snap01 /dev/vg0/mongodb

This command creates an LVM snapshot (with the --snapshot option) named mdb-snap01 of the mongodb volume in the vg0 volume group.

This example creates a snapshot named mdb-snap01 located at /dev/vg0/mdb-snap01. The location and paths to your systems volume groups and devices may vary slightly depending on your operating system’s LVM configuration.

The snapshot has a cap of at 100 megabytes, because of the parameter --size 100M. This size does not reflect the total amount of the data on the disk, but rather the quantity of differences between the current state of /dev/vg0/mongodb and the creation of the snapshot (i.e. /dev/vg0/mdb-snap01.)

Warning

Ensure that you create snapshots with enough space to account for data growth, particularly for the period of time that it takes to copy data out of the system or to a temporary image.

If your snapshot runs out of space, the snapshot image becomes unusable. Discard this logical volume and create another.

The snapshot will exist when the command returns. You can restore directly from the snapshot at any time or by creating a new logical volume and restoring from this snapshot to the alternate image.

While snapshots are great for creating high quality backups very quickly, they are not ideal as a format for storing backup data. Snapshots typically depend and reside on the same storage infrastructure as the original disk images. Therefore, it’s crucial that you archive these snapshots and store them elsewhere.

Archive a Snapshot

After creating a snapshot, mount the snapshot and move the data to separate storage. Your system might try to compress the backup images as you move the offline. The following procedure fully archives the data from the snapshot:

umount /dev/vg0/mdb-snap01
dd if=/dev/vg0/mdb-snap01 | gzip > mdb-snap01.gz

The above command sequence does the following:

  • Ensures that the /dev/vg0/mdb-snap01 device is not mounted.

  • Performs a block level copy of the entire snapshot image using the dd command and compresses the result in a gzipped file in the current working directory.

    Warning

    This command will create a large gz file in your current working directory. Make sure that you run this command in a file system that has enough free space.

Restore a Snapshot

To restore a snapshot created with the above method, issue the following sequence of commands:

lvcreate --size 1G --name mdb-new vg0
gzip -d -c mdb-snap01.gz | dd of=/dev/vg0/mdb-new
mount /dev/vg0/mdb-new /srv/mongodb

The above sequence does the following:

  • Creates a new logical volume named mdb-new, in the /dev/vg0 volume group. The path to the new device will be /dev/vg0/mdb-new.

    Warning

    This volume will have a maximum size of 1 gigabyte. The original file system must have had a total size of 1 gigabyte or smaller, or else the restoration will fail.

    Change 1G to your desired volume size.

  • Uncompresses and unarchives the mdb-snap01.gz into the mdb-new disk image.

  • Mounts the mdb-new disk image to the /srv/mongodb directory. Modify the mount point to correspond to your MongoDB data file location, or other location as needed.

Note

The restored snapshot will have a stale mongod.lock file. If you do not remove this file from the snapshot, and MongoDB may assume that the stale lock file indicates an unclean shutdown. If you’re running with storage.journal.enabled enabled, and you do not use db.fsyncLock(), you do not need to remove the mongod.lock file. If you use db.fsyncLock() you will need to remove the lock.

Restore Directly from a Snapshot

To restore a backup without writing to a compressed gz file, use the following sequence of commands:

umount /dev/vg0/mdb-snap01
lvcreate --size 1G --name mdb-new vg0
dd if=/dev/vg0/mdb-snap01 of=/dev/vg0/mdb-new
mount /dev/vg0/mdb-new /srv/mongodb

Remote Backup Storage

You can implement off-system backups using the combined process and SSH.

This sequence is identical to procedures explained above, except that it archives and compresses the backup on a remote system using SSH.

Consider the following procedure:

umount /dev/vg0/mdb-snap01
dd if=/dev/vg0/mdb-snap01 | ssh username@example.com gzip > /opt/backup/mdb-snap01.gz
lvcreate --size 1G --name mdb-new vg0
ssh username@example.com gzip -d -c /opt/backup/mdb-snap01.gz | dd of=/dev/vg0/mdb-new
mount /dev/vg0/mdb-new /srv/mongodb

Create Backups on Instances that do not have Journaling Enabled

If your mongod instance does not run with journaling enabled, or if your journal is on a separate volume, obtaining a functional backup of a consistent state is more complicated. As described in this section, you must flush all writes to disk and lock the database to prevent writes during the backup process. If you have a replica set configuration, then for your backup use a secondary which is not receiving reads (i.e. hidden member).

  1. To flush writes to disk and to “lock” the database (to prevent further writes), issue the db.fsyncLock() method in the mongo shell:

    db.fsyncLock();
    
  2. Perform the backup operation described in Create a Snapshot.

  3. To unlock the database after the snapshot has completed, use the following command in the mongo shell:

    db.fsyncUnlock();
    

    Note

    Changed in version 2.0: MongoDB 2.0 added db.fsyncLock() and db.fsyncUnlock() helpers to the mongo shell. Prior to this version, use the fsync command with the lock option, as follows:

    db.runCommand( { fsync: 1, lock: true } );
    db.runCommand( { fsync: 1, lock: false } );
    

    The database cannot be locked with db.fsyncLock() while profiling is enabled. You must disable profiling before locking the database with db.fsyncLock(). Disable profiling using db.setProfilingLevel() as follows in the mongo shell:

    db.setProfilingLevel(0)
    

    Changed in version 2.2: When used in combination with fsync or db.fsyncLock(), mongod may block some reads, including those from mongodump, when queued write operation waits behind the fsync lock.