[SERVER-34482] system.indexes 3.4 to 3.6 upgrade semantics on wiredTiger may cause mongod 4.0 to fail to start up Created: 13/Apr/18  Updated: 29/Oct/23  Resolved: 30/Apr/18

Status: Closed
Project: Core Server
Component/s: Upgrade/Downgrade
Affects Version/s: None
Fix Version/s: 4.0.0-rc0

Type: Bug Priority: Major - P3
Reporter: Maria van Keulen Assignee: Maria van Keulen
Resolution: Fixed Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Depends
Backwards Compatibility: Fully Compatible
Operating System: ALL
Sprint: Storage NYC 2018-05-07
Participants:
Linked BF Score: 0

 Description   

system.indexes does not receive a UUID upon setFCV from 3.4 to 3.6 regardless of storage engine. Mongod 4.0 only exempts system.indexes from the UUID startup check if the storage engine is MMAP. This means that if a user creates system.indexes on mongod 3.4 or earlier when using wiredTiger, upgrades to mongod 3.6, then upgrades to mongod 4.0, mongod 4.0 will fail to startup.

One solution to avoid this issue is to drop system.indexes on startup if it fails the UUID check.



 Comments   
Comment by Githook User [ 01/May/18 ]

Author:

{'email': 'maria@mongodb.com', 'name': 'Maria van Keulen', 'username': 'mvankeulen94'}

Message: SERVER-34482 Don't run test on MMAP
Branch: master
https://github.com/mongodb/mongo/commit/cd5653b7c9ab89db8c1770714b269b7b00609551

Comment by Githook User [ 30/Apr/18 ]

Author:

{'email': 'maria@mongodb.com', 'username': 'mvankeulen94', 'name': 'Maria van Keulen'}

Message: SERVER-34482 Drop MMAP-specific system collections on wiredTiger

It is possible for a user to explicitly create MMAP-specific system
collections when using wiredTiger. The presence of these collections
may cause UUID checks to fail.
Branch: master
https://github.com/mongodb/mongo/commit/7a2217a54d59c5d97e9e79cc40639c2589a18deb

Comment by Maria van Keulen [ 24/Apr/18 ]

In light of SERVER-32894, I think we should consider dropping system.indexes and system.namespaces on wiredTiger regardless of whether they pass the UUID check.

Comment by Maria van Keulen [ 13/Apr/18 ]

I do not believe it is possible to create system.namespaces on (non-MMAP) mongod 3.4, but if it is possible for system.namespaces to exist on a non-MMAP mongod 3.4 node, system.namespaces will have this issue as well.

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