mongodb - Upgrading older mongo database after unintentional mongo version upgrade

Question

Is there any way to upgrade a mongo database after the mongodb package was unintentionally upgraded (3.4.9 -> 3.6.1)?

According to the mongo docs, as a prerequisite of the upgrade, featureCompatibilityVersion has to be set from the mongo cli tool, which, however, needs a running mongod database daemon, which, however, won't run if mongodb package was already upgraded and the database was not set the featureCompatibilityVersion flag.

I'm on ArchLinux, I had mongodb in IgnorePkg but it's dependencies boost-libs and wiredtiger got upgraded anyways. Having pacman cache already wiped out, I compiled older versions of the packages myself, but running mongodb keeps failing with this error:

unsupported WiredTiger file version: this build  only supports major/minor versions up to 1/0,  and the file is version 2/0: WT_ERROR: non-specific WiredTiger error

I don't have a clue what the hell this means (the problematic version of wiredtiger is 2.9.3-1, version after the upgrade is 2.9.3.20171205-2).

Seems like I can neither downgrade nor upgrade...

Other cli tools (e.g. mongodump) also won't run without a running database, is there any other possibility (some wiredtiger related tool)? Deleting the database and starting afresh is not an option.

Answer 1

I solved the problem by installing the older version of mongodb on a system for which it wasn't a problem to get it pre-packaged (well, Windows 10, even though oficially it's stated the package is for Windows Server 2008), copying the database files (contents of /var/lib/mongodb) there, running it with --dbpath param (mongod --dbpath /path/to/dbfiles), setting the compatibility flag according to the docs and finally copying the db files back to the server.

Would like to know about a better option, but it's good to know the db files are easily transferable to another system, even another architecture (the db was relatively simple and small though).