How do I migrate a large db from zlib to zstd?

[chris (Chris Dellaway)]

@Eric_Miller

Can you elaborate on how an initial sync gets me to zstd from zlib? It is my understanding that the initial sync is going to use the same creation scheme as the source, but maybe there is a way to change that?

The storage engine of the node will determine its own compression. So the instance you are reinitializing should have the compressors configured.

Stop the node to update. Remove the data files. Add your desired compressor configuration. Start the node.

With a set of sampledata you can observe the difference with the third node with the blockcompressor enabled.

All of these are brand new replicaset members before chaning the 3rd node.

$ for l in a b c ; do mongo --quiet --host mongo-0-${l} --eval "db.getMongo().setSecondaryOk(); db.adminCommand('listDatabases').totalSize" ; done
159854592
159002624
107610112

@Eric_Miller

If a new initial sync is the right way to get to zstd, then how can I speed it up (and I mean drastically)? Setting the read preference isn’t going to make that much difference and at the current rate I’d need a HUGE oplog, maybe even bigger than my database.

The manual says it starts the oplog copy when the initial sync starts. So I don’t think that will be an issue.
But this is a lot of data to scan from compressed blocks and transmit. I don’t have any tricks to speed this up.