Using rs.syncFrom for initial sync

[Ralf Kistner]

To reduce the impact on my primary node when adding a new secondary, I'd like it to perform the initial sync from an existing secondary.

The rs.syncFrom command seems perfect for that:

http://docs.mongodb.org/manual/tutorial/configure-replica-set-secondary-sync-target/

Note however, that the page mentions:

To affect the sync target for the initial sync, run rs.syncFrom() operation before initial sync.

 

My question is, how do I run rs.syncFrom before the initial sync? If I run it before the new member is added to the replica set, I get the error "can't get local.system.replset config from self or any seed (EMPTYCONFIG)". If I add the member, it immediately starts the initial sync, and I can't run the command anymore.

Regards,

Ralf Kistner

[Joanna Cheng]

Hi Ralf,

There is actually a gap between adding a node to a replica set, and the actual initial sync process starting. During this gap, you can change the sync target using the rs.syncFrom() command. On my machine, this takes 40 seconds; however this will vary depending on your system, the type of disks you are running, how long space allocation for the oplog takes, etc. However, once the initial sync starts, you cannot change the sync target.

You can tell when the initial sync starts when you see the following lines in your mongod.log

Mon Mar 24 10:21:21.231 [rsSync] replSet initial sync pending

Mon Mar 24 10:21:21.231 [rsSync] replSet syncing to: XXXXX:27017

An alternative to this would be to sync by copying files from another member

Kind regards,

Joanna

[Ralf Kistner]

Thanks Joanna, that explains it. Copying the files would be a better solution for us for now.

Regards,

Ralf

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