changing mongod dbpath raise permission error - mongodb

Question

I needed more space on my mongo database so I went to my local computer shop, bought a new hard drive, installed it. Then I copied all the files from /var/lib/mongodb to /media/didier/mongodb/mongodb/ and changed the ownership of /media/didier/mongodb/mongodb/ with the command sudo chown -R mongodb:mongodb /media/didier/mongodb/mongodb/. Then, I changed the config file at /etc/mongod.conf to point to the good data directory. Basically, I followed the instruction at Changing MongoDB data store directory.
The permissions looks good, the config file looks fine too, but when I start mongodb using sudo service mongod start I have the following exception in my logs
014-07-16T09:11:37.362-0400 [initandlisten] MongoDB starting : pid=5899 port=27017
dbpath=/media/didier/mongodb/mongodb/ 64-bit host=miaou
2014-07-16T09:11:37.362-0400 [initandlisten] db version v2.6.3
2014-07-16T09:11:37.362-0400 [initandlisten] git version: 255f67a66f9603c59380b2a389e386910bbb52cb
2014-07-16T09:11:37.362-0400 [initandlisten] build info: Linux build12.nj1.10gen.cc 2.6.32-431.3.1.el6.x86_64 #1 SMP Fri Jan 3 21:39:27 UTC 2014 x86_64 BOOST_LIB_VERSION=1_49
2014-07-16T09:11:37.362-0400 [initandlisten] allocator: tcmalloc
2014-07-16T09:11:37.362-0400 [initandlisten] options: { config: "/etc/mongod.conf", net: { bindIp: "127.0.0.1" }, storage: { dbPath: "/media/didier/mongodb/mongodb/" }, systemLog: { destination: "file", logAppend: true, path: "/var/log/mongodb/mongod.log" } }
2014-07-16T09:11:37.362-0400 [initandlisten] exception in initAndListen std::exception: boost::filesystem::status: Permission denied: "/media/didier/mongodb/mongodb/", terminating
2014-07-16T09:11:37.362-0400 [initandlisten] dbexit:
2014-07-16T09:11:37.362-0400 [initandlisten] shutdown: going to close listening sockets...
2014-07-16T09:11:37.362-0400 [initandlisten] shutdown: going to flush diaglog...
2014-07-16T09:11:37.362-0400 [initandlisten] shutdown: going to close sockets...
2014-07-16T09:11:37.362-0400 [initandlisten] shutdown: waiting for fs preallocator...
2014-07-16T09:11:37.362-0400 [initandlisten] shutdown: lock for final commit...
2014-07-16T09:11:37.362-0400 [initandlisten] shutdown: final commit...
2014-07-16T09:11:37.362-0400 [initandlisten] shutdown: closing all files...
2014-07-16T09:11:37.362-0400 [initandlisten] closeAllFiles() finished
2014-07-16T09:11:37.362-0400 [initandlisten] dbexit: really exiting now
Any idea what goes wrong? When I simply run sudo mongod --config /etc/mongod.conf everything works fine. So I suppose it is a permission problem.
cheers!

Answer 1

The problem is that the hard drive is not accessible to the mongodb user. To share a hard drive between all users, you need to change the /etc/fstab by adding the following line:
UUID=a43bc179-f0e5-464b-bd77-1be6a7581ec2 /media/mongodb ext4 defaults 0 0
That will mount the hard drive to the /media/mongodb directory at boot time. See https://help.ubuntu.com/community/Fstab for more information.
In short, to use a different hard drive as data location on mongodb on a ubuntu machine one needs to do the following three steps:
make sure the mongodb user can access the new hard drive (fstab)
make sure the user mongodb has access to the data directory (chown)
change the mongo config file (/etc/mongod.conf)

Answer 2

I think the issue is you need to change permissions of your /media mount point to have r+x (read and execute) for 'other' groups and users; at least that fixed it for me. Doing
sudo chmod a+rx /media
will do it. You want to make sure this is also true for the whole path to the mongodb data I think, so you could do sudo chmod -R a+rx /media to make it recursive, or change the permissions for the drive, then each subfolder up to the mongodb folder:
sudo chmod a+rx /media/didier
sudo chmod a+rx /media/didier/mongodb
sudo chmod a+rx /media/didier/mongodb/mongodb