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MongoDB 2.2: why didn't replication catch up a collection following a dump/restore?

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.

Solution

  • 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/