[DOCS-15986] Investigate changes in SERVER-74214: Cannot downgrade from fresh 6.2 due to oplog cap size Created: 25/Mar/23  Updated: 13/Nov/23  Resolved: 03/Aug/23

Status: Closed
Project: Documentation
Component/s: manual, Server
Affects Version/s: None
Fix Version/s: 7.0.0-rc0, Server_Docs_20231030, Server_Docs_20231106, Server_Docs_20231105, Server_Docs_20231113

Type: Task Priority: Major - P3
Reporter: Backlog - Core Eng Program Management Team Assignee: David Hou
Resolution: Done Votes: 0
Labels: None
Σ Remaining Estimate: Not Specified Remaining Estimate: Not Specified
Σ Time Spent: Not Specified Time Spent: Not Specified
Σ Original Estimate: Not Specified Original Estimate: Not Specified

Issue Links:
Documented
documents SERVER-74214 Cannot downgrade from fresh 6.2 due t... Closed
Sub-Tasks:
Key
Summary
Type
Status
Assignee
DOCS-16284 6.0 specific updates Sub-task Closed David Hou  
Participants:
Days since reply: 17 weeks ago
Story Points: 2

 Description   
Original Downstream Change Summary

https://www.mongodb.com/docs/manual/core/capped-collections/#create-a-capped-collection is out of date.
Needs rework:
"If the size field is less than or equal to 4096, then the collection will have a cap of 4096 bytes. Otherwise, MongoDB will raise the provided size to make it an integer multiple of 256."

Description of Linked Ticket

SERVER-67246 unlocked the ability for the oplog capped size to be anything, not just a multiple of 256. This blocks fresh installs from downgrading.
I think we should fix this by adjusting the oplog cap size algorithm to go back to rounding to the nearest 256. This will avoid problems with on-prem LTS users on 7.0 downgrading to 6.0.



 Comments   
Comment by David Hou [ 11/Oct/23 ]

matt.kneiser@mongodb.com Thanks for the update - feel free to create a new DOCS ticket.

Comment by Matt Kneiser [ 11/Oct/23 ]

david.hou@mongodb.com I was mistaken during this docs review. The relevant feature flag has been enabled by default since 6.2 when this code landed in master. In 6.2+, non-oplog capped collections do not get rounded. Do you want to make a new PR under this ticket or make a new DOCS ticket to reflect this? Up to you.

Comment by David Hou [ 26/Jul/23 ]

matt.kneiser@mongodb.com When you get a chance, can you review this small PR and confirm if this is the only necessary update? Thanks!

https://github.com/10gen/docs-mongodb-internal/pull/4158

Comment by David Hou [ 26/Jul/23 ]

SERVER-74214 adjusts the oplog cap size algorithm back to rounding to the nearest 256. This is already reflected in the documentation for v7.0: https://www.mongodb.com/docs/v7.0/core/capped-collections/#create-a-capped-collection

Based on the comment thread for SERVER-74214, looks like the only update is to add a note to downgrade docs to mention that we will not support downgrading from fresh installs.

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