Is it safe to run mongodump against running server with many writes per second? Is it possible to get corrupted dump doing in this way?
From here:
Use --oplog to capture incoming write operations during the mongodump operation to ensure that the backups reflect a consistent data state.
Does it mean that no matter how many writes in database dump will be consistent?
If I ran mongodump --oplog at 1AM and it finished at 2AM then I run mongorestore --oplogReplay what state will I get?
From here:
However, the use of mongodump and mongorestore as a backup strategy can be problematic for sharded clusters and replica sets.
but why? I had replica set of 1 primary and 2 secondary. What the problem to run mongodump against one of secondary? It should same as primary (except replication lag difference).
The docs are quite clear about it:
--oplog
Creates a file named oplog.bson as part of the mongodump output. The oplog.bson file, located in the top level of the output directory, contains oplog entries that occur during the mongodump operation. This file provides an effective point-in-time snapshot of the state of a mongod instance. To restore to a specific point-in-time backup, use the output created with this option in conjunction with mongorestore --oplogReplay.
Without --oplog, if there are write operations during the dump operation, the dump will not reflect a single moment in time. Changes made to the database during the update process can affect the output of the backup.
--oplog has no effect when running mongodump against a mongos instance to dump the entire contents of a sharded cluster. However, you can use --oplog to dump individual shards.
Without --oplog you still get a valid dump, just a bit inconsistent - some of the writes done between 1 AM and 2 AM will be missing.
With --oplog you have the oplog file captured at 2 AM. The dump remains inconsistent, and replaying the oplog on restore fixes this issue.
The problems dumping the sharded clusters deserve a dedicated page in the docs. Essentially because of complexity to synchronise backups of all nodes:
To create backups of a sharded cluster, you will stop the cluster balancer, take a backup of the config database, and then take backups of each shard in the cluster using mongodump to capture the backup data. To capture a more exact moment-in-time snapshot of the system, you will need to stop all application writes before taking the filesystem snapshots; otherwise the snapshot will only approximate a moment in time.
There are no problems to dump replica set.
Related
I'm a SQL Server DBA trying to use MongoDB for some particular cases. What I have at the moment is 3 node replica set with 2 data bearing nodes and 1 arbiter. The thing that i'm struggling at the moment is finding a clear answer on ho to create Backups that will allow me a point in time restores, similar to what you have in MS SQL with FULL and LOG backups. How can i do that?
Mongodb provides different methods to Backup and restore
1. Back Up with Atlas(cloud based AWS services)
2. Back Up with MongoDB Cloud Manager or Ops Manager(Enterprise edition only. Supports backing up and restoring MongoDB replica sets and sharded clusters from a graphical user interface.)
3. Back Up with file system snapshot on OS( on Linux, the Logical Volume Manager (LVM) can create snapshots. Similarly, Amazon’s EBS storage system for EC2 supports snapshots)
To get a correct snapshot of a running mongod process, you must have journaling enabled. Without journaling enabled, there is no guarantee that the snapshot will be consistent or valid.
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.
To restore a snapshot , issue 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
for more details https://docs.mongodb.com/manual/tutorial/backup-with-filesystem-snapshots/#back-up-and-restore-using-lvm-on-linux
4.Back Up with mongodump (Terminal based Mongodb tools)
mongodump and mongorestore are simple and efficient tools for backing up and restoring small MongoDB deployments
mongodump and mongorestore operate against a running mongod process
If you dont specify any database it captures all the databases and copies into seperate folder along with indexes(json format) for every database
By default, mongodump does not backup the local database(which contains Replicaset configuration & oplog.rs collection).
For replica sets, mongodump provides the --oplog option to include in its output oplog entries that occur during the mongodump operation. This allows the corresponding mongorestore operation to replay the captured oplog. To restore a backup created with --oplog, use mongorestore with the --oplogReplay option.
Mongorestore captures only database files. indexes must be rebuild after restoring the data.
https://docs.mongodb.com/manual/tutorial/backup-and-restore-tools/#
commands:
mongodump --out /data/backup/ (It backups all the databases and indexes)
mongodump --collection myCollection --db test (specified database & collection)
mongorestore --port
We have a three-server replicaset running MongoDB 2.2 on Ubuntu 10.04, and recently had to upgrade the hard drive for each server where one particular database resides. This database contains log information for web service requests, where they write to collections in hourly buckets using the current timestamp to determine the name, e.g. log_yyyymmddhh.
I performed this process:
backup the database on the primary server with mongodump --db log_db
take a secondary server offline, replace the disk
bring the secondary server up in standalone mode (i.e. comment out the replSet entry
in /etc/mongodb.conf before starting the service)
restore the database on the secondary server with mongorestore --drop --db log_db
add the secondary server back into the replicaset and bring it online,
letting replication catch up the hourly buckets that were updated/created
while it had been offline
Everything seemed to go as expected, except that the collection which was the current bucket at the time of the backup was not brought up to date by replication. I had to manually copy that collection over by hand to get it up to date. Note that collections which were created after the backup were synched just fine.
What did I miss in this process that caused MongoDB not to get things back in synch for that one collection? I assume something got out of whack with regard to the oplog?
Edit 1:
The oplog on the primary showed that its earliest timestamp went back a couple of days, so there should have been plenty of space to maintain transactions for a few hours (which was the time the secondary was offline).
Edit 2:
Our MongoDB installation uses two disk partitions: /dev/sda1 and /dev/sdb1. The primary MongoDB directory /var/lib/mongodb/ is on /dev/sda1, and holds several databases, while the log database resides by itself on /dev/sdb1. There's a sym link /var/lib/mongodb/log_db which points to a directory on /dev/sdb1. Since the log db was getting full, we needed to upgrade the disk for /dev/sdb1.
You should be using mongodump with the --oplog option. Running a full database backup with mongodump on a replicaset that is updating collections at the same time may not leave you with a consistent backup. This becomes worse with larger databases, more collections and more frequent updates/inserts/deletes.
From the documentation for your version (2.2) of MongoDB (it's the same for 2.6 but just to be as accurate as possible):
--oplog
Use this option to ensure that mongodump creates a dump of the
database that includes an oplog, to create a point-in-time snapshot of
the state of a mongod instance. To restore to a specific point-in-time
backup, use the output created with this option in conjunction with
mongorestore --oplogReplay.
Without --oplog, if there are write operations during the dump
operation, the dump will not reflect a single moment in time. Changes
made to the database during the update process can affect the output
of the backup.
http://docs.mongodb.org/v2.2/reference/mongodump/
This is not covered well in most MongoDB tutorials around backups and restores. Generally you are better off if you can perform a live snapshot of the storage volume your database resides on (assuming your storage solution has a live snapshot ability compatible with MongoDB). Failing that, your next best bet is taking a secondary offline and then performing a snapshot or backup of the database files. Mongodump on a live database is increasingly a less optimal solution for larger databases due to performance issues.
I'd definitely take a look at the MongoDB overview of backup options: http://docs.mongodb.org/manual/core/backups/
I would guess this has to do with the oplog not being long enough, although it seems like you checked that and it looked reasonably big.
Still, when adding new members to a replica set you shouldn't be snapshotting and restoring them. It's better to simply add a new member and let replication happen by itself. This is described in the Mongo docs and is the process I've always followed.
I have a standalone MongoDb instance. It has many databases in it. I am though only concerned with backingup/restoring one of those databases, lets call it DbOne.
Using the instructions in (http://www.mongodb.com/blog/post/dont-let-your-standalone-mongodb-server-stand-alone), I can create an oplog on this standalone server.
Using the tool Tayra, I can record/store the oplog entries. Being able to create incremental backups is the main reason I enabled the oplog on my standalone instance.
I intend to take full backups once a day, using the command
mongodump --db DbOne --oplog
From my understanding, this backup will contain a point-in-time snapshot of my db.
Assuming I want to discard all updates since this backup, I delete all backedup oplog and I restore only this full backup, using the command
mongorestore --drop --db DbOne --oplogReplay
At this point, do I need to do something to the oplog collection in the local db? Will mongodb automatically drop the entries pertaining to this db from the oplog? Because if not, then wouldn't Tayra end up finding those oplog entries and backup them all over again?
Tbh, I haven't tried this yet on my machine. I am hoping someone can point to a document that lists supported/expected behaviour in this scenario.
I had experimented with a MongoDb server, setup as a replica set with only 1 member, shortly after asking the question. I however forgot to answer this question.
I took a backup using mongodump --db DbOne --oplog. I executed some additional updates. Keeping the mongodb server as is, ie still running under replication, if I run the mongorestore command, then it would create thousands of oplog entries, one for each document of each collection in the db. It was a big mess.
The alternative was to shutdown MongoDb and start it as a standalone instance (ie not running as a replica set). Now if I were to restore using mongorestore the oplog wouldnt be touched. This was bad, because the oplog now contained entries that were not present in the actual collections.
I wanted a mechanism that would restore both my database as well as oplog info in the local database to the time the backup took place. mongodump doesnt backup the local database.
Eventually I had stop using mongodump and instead switched to backing up the entire data directory (after stopping mongodb). Once we switch to AWS, I could use the EBS Snapshot feature to perform the same.
I understand you want a link to the docs about mongorestore:
http://docs.mongodb.org/manual/reference/program/mongorestore/
From what I understand you want to make a point in time backup and then restore that backup. The commands you listed above will do that:
1)mongodump --db DbOne --oplog
2)mongorestore --drop --db DbOne --oplogReplay
However, please note that the "point in time" that the backup was effectively taken at it is when the dump ends, not the moment the command started. This is a fine detail that might not matter to you, but is included for completeness.
Let me know if there is anything else.
Best,
Charlie
I'm looking for the right way to do a Mongodb backup on a replica set (non-sharded).
By reading the Mongodb documentation, I understand that a "mongodump --oplog" should be enough, even on a replica (slave) server.
From the mongodb / mongodump documentation :
--oplog
Use this option to ensure that mongodump creates a dump of the database that includes an oplog, to create a point-in-time snapshot of the state of a mongod instance. To restore to a specific point-in-time backup, use the output created with this option in conjunction with mongorestore --oplogReplay.
Without --oplog, if there are write operations during the dump operation, the dump will not reflect a single moment in time. Changes made to the database during the update process can affect the output of the backup
I'm still having a very hard time to understand how Mongodb can backup and keep writing on the database and make a consistent backup, even with --oplog.
Should I lock my collections first or is it safe to run "mongodump --oplog ?
Is there anything else I should know about?
Thanks.
The following document explains how mongodump with –oplog option works to create a point in time backup.
http://docs.mongodb.org/manual/tutorial/backup-databases-with-binary-database-dumps/
However, using mongodump and mongorestore to back up and restore MongodDB can be slow. If file system snapshot is an option, you may want to consider using snapshot for MongoDB backup. Information from the following link details two snapshot options for performing hot backup of MongoDB.
http://docs.mongodb.org/manual/tutorial/backup-databases-with-filesystem-snapshots/
You can also look into MongoDB backup service.
http://www.10gen.com/products/mongodb-backup-service
I'm in the middle of setting up a backup strategy for mongo, was just curious to know if mongodump locks the database before performing the database dump?
I found this on mongo's google group:
Mongodump does a simple query on the live system and does not require
a shutdown. Like all queries it requires a read lock while running but
doesn't not block any more than normal queries.
If you have a replica set you will probably want to use the --oplog
flag to do your backups.
See the docs for more information
http://docs.mongodb.org/manual/administration/backups/
Additionally I found this previously asked question
MongoDB: mongodump/restore vs. backup up files directly
Excerpt from above question
Locking and copying files is only an option when you don't have heavy
write load.
mongodump can be run against live server. It will create some
additional load, so don't do it on peak hours. Also, it is advised to
do it on a secondary node (if you don't use replica sets, you should).
There are some complications when you have a DB so large that no
single machine can hold it. See this document.
Also, if you have replica set, you take down one of secondaries and copy its files directly. See http://www.mongodb.org/display/DOCS/Backups:
Mongdump does not lock the db. It means other read and write operations will continue normally.
Actually, both mongodump and mongorestore are non-blocking. So if you want to mongodump mongorestore a db then its your responsibility to make sure that it is really a desired snapshot backup/restore. To do this, you must stop all other write operations while taking/restoring backups with mongodump/mongorestore. If you are running a sharded environment then its recommended you stop the balancer also.