MongoDB could not restore backup because of “key too large to index” error

Recently I made a db dump on a MongoDB 2.4 server like this:

mongodump --db veye_dev

And I tried to restore it on a MongoDB 2.6 server like this:

mongorestore dump/veye_dev

Unfortunately at some point the restore process failed with this error message:

mongo error: "Btree::insert: key too large to index, failing

There are some restrictions for the size of the index in MongoDB 2.6. Luckily I found the issue on MongoDBs ticket system. The import can succeed if the mongod process is running with the parameter “failIndexKeyTooLong=false”. Just start it like that:

sudo mongod --setParameter failIndexKeyTooLong=false

And now execute the mongorestore again. That worked for me.

Published by Robert Reiz

CEO @ VersionEye. Passionated software developer since 1998.

7 thoughts on “MongoDB could not restore backup because of “key too large to index” error

  1. This just bit me too. I also discovered that you can set that flag on a running mongod instance, with:

    db.getSiblingDB(‘admin’).runCommand( { setParameter: 1, failIndexKeyTooLong: false } )

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