Mongodb – check storage engine from shell

Have you tried using the sh.startBalancer() helper instead?

Rather than a straight update, it does takes an timeout argument as how long to wait for balancing to start as well as a sleep interval in terms of how long to sleep between waiting. Here's the code from the shell by way of explanation:

mongos> sh.startBalancer
function ( timeout, interval ) {
    sh.setBalancerState( true )
    sh.waitForBalancer( true, timeout, interval )
}

So, you could even break it up and use the waitForBalancer helper if you wished. For reference, here is the equivalent stopBalancer command erroring out when I tried to stop it with a config server down:

mongos> sh.stopBalancer(2000, 100)
Waiting for active hosts...
Waiting for active host adamc-mbp.local:30999 to recognize new settings... (ping : Tue Dec 31 2013 19:51:32 GMT+0000 (GMT))
Waiting for the balancer lock...
Waiting again for active hosts after balancer is off...
Tue Dec 31 19:51:39.243 error: {
    "$err" : "error creating initial database config information :: caused by :: SyncClusterConnection::udpate prepare failed:  localhost:29000:9001 socket exception [FAILED_STATE] server [localhost:29000] ",
    "code" : 8005
} at src/mongo/shell/query.js:128

It looks like the current MongoDB 3.0 upgrade instructions are missing mention of two important parameters for backing up and restoring users and roles:

• mongodump --dumpDbUsersAndRoles (see also: Required Access to Backup User Data).
• mongorestore --restoreDbUsersAndRoles (see also: Required Access to Restore User Data)

I can think of several approaches to fix:

1. If you don't have many user accounts on the config servers, recreate the administrator & user accounts. This isn't ideal, but is probably the fastest approach.

2. Export the users from your mmap database. This is more involved, but saves you recreating the users & roles. I've described steps for this below.

3. Redo the config server migration with the user & role information included. I expect this is the least desirable option.

Exporting the users

Assuming you have already upgraded all of your config servers to WiredTiger, here are some steps to add the user information:

1. Disable the balancer

2. Stop the last config server listed in your mongos' configDB setting (will call that config3 for the purpose of these steps). This will ensure your sharded cluster metadata remains read-only for the following steps.

3. Re-start config2 using the mmap data directory

   At this stage you should have:

   • config1 (running WiredTiger)
   • config2 (running mmap with user/role data)
   • config3 (stopped)

4. Export the data from config2:

   mongodump --db config --dumpDbUsersAndRoles --username .. --password ..

   Add any other parameters needed, eg --authenticationDatabase .. if you need to auth against another database.

5. If you have users in the admin database on your config server, you will also want to dump that as well.

6. (optional) Remove files from your dump except for the user/role information. If you are certain nothing has changed since you did the original migration from mmap to WiredTiger you could skip this step, however it would be safer to not overwrite any existing data.

   Preview the files to remove:

   find ./dump -type f -not -name "\$admin.system*"

   WARNING: removing files, make sure you have previewed to confirm:

   find ./dump -type f -not -name "\$admin.system*" | xargs rm

7. Re-start config2 using the wiredTiger storage engine

8. Run: mongorestore --db config --restoreDbUsersAndRoles dump/config/

   You should see messages about restoring users & roles, for example:

   2015-03-18T02:41:34.887+1100 restoring users from dump/config/$admin.system.users.bson

   2015-03-18T02:41:34.887+1100 restoring roles from dump/config/$admin.system.roles.bson

9. Login to config2 and confirm the users are correctly setup (i.e. auth with admin account, use db.getUsers() to check).

   At this stage you should have:

   • config1 (running WiredTiger)
   • config2 (running WiredTiger with user/role data)
   • config3 (stopped)

10. Copy the dump directory to config1 and repeat the mongorestore step.

11. Shutdown config2 (to keep the sharded cluster metadata readonly for the next step).

    At this stage you should have:

    • config1 (running WiredTiger with user/role data)
    • config2 (stopped)
    • config3 (stopped)

12. Start config3. Copy the dump directory to config3, and repeat the mongorestore step.

    At this stage you should have:

    • config1 (running WiredTiger with user/role data)
    • config2 (stopped)
    • config3 (running WiredTiger with user/role data)

13. Start config2. At this point all config servers should be online with the user information.

14. Re-enable the balancer so normal balancing activity & chunk migration can resume.