[SERVER-86290] Resharding/moveCollection machinery does not support capped collections Created: 06/Feb/24  Updated: 06/Feb/24

Status: Needs Scheduling
Project: Core Server
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Task Priority: Major - P3
Reporter: Pierlauro Sciarelli Assignee: Backlog - Catalog and Routing
Resolution: Unresolved Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Assigned Teams:
Catalog and Routing
Participants:

 Description   

When trying to reshard/move a capped collection, the resharding recipient service hits an unrecoverable error because snapshot reads are not supported on capped collections. The restriction seems to have been introduced because truncate is used on capped collections and it can potentially break snapshot reads.



 Comments   
Comment by Max Hirschhorn [ 06/Feb/24 ]

The restriction seems to be due to truncate being used on capped collections.

Dan Gottlieb clarified in the subsequent comment how capped collections do not use WT's truncate to do the deletion. The issue for snapshot reads is that deletes weren't happening at a point-in-time due to not being replicated. SERVER-16049 changed the behavior where capped deletions are now replicated. Furthermore, SERVER-56230 was meant to remove the restriction and thus enable running the dbHash command at a point-in-time in a snapshot read. CC gregory.wlodarek@mongodb.com

pierlauro.sciarelli@mongodb.com, can you clarify the ask for Cluster Scalability? As far as I understand this limitation for capped collections and snapshot reads is artificial and either Catalog and Routing or Storage Execution would be responsible for removing it.

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