[DOCS-15700] Investigate changes in SERVER-65289: Improvements to Mirrored Reads Metrics and Visibility Created: 24/Oct/22  Updated: 13/Nov/23  Resolved: 22/Feb/23

Status: Closed
Project: Documentation
Component/s: manual, Server
Affects Version/s: None
Fix Version/s: 6.1.0-rc0, Server_Docs_20231113

Type: Task Priority: Major - P3
Reporter: Backlog - Core Eng Program Management Team Assignee: Kenneth Dyer
Resolution: Fixed Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Documented
documents SERVER-65289 Improvements to Mirrored Reads Metric... Closed
Participants:
Days since reply: 1 year, 15 weeks, 2 days ago
Epic Link: DOCSP-21764

 Description   
Original Downstream Change Summary

This ticket adds new serverStatus metrics for the mirrored reads section (https://www.mongodb.com/docs/manual/reference/command/serverStatus/#mirroredreads).

The new user-facing field is mirroredReads.processedAsSecondary:

  • the number of mirrored reads processed successfully by a secondary.

    Description of Linked Ticket

    Hi Team,

Starting in v4.4 we introduced the Mirrored Reads feature, and we have a couple of server metrics tracking that:

  • ss mirroredReads seen: tracks the number of operations that can be mirrored ("mirrorable").
  • ss mirroredReads sent: tracks the numbe of "mirrorable" operations that were actually sent to Secondaries.

The problem is that there is no indication in the Secondaries' Diagnostic Data or Logs of the amount of reads executed as a result of the Mirror Reads feature.

For Troubleshooting purposes, it would be nice to have the following:

  1. When an instance is a Secondary and it receives mirrored reads, track them in the Diagnostic Data with a metric similar to ss mirroredReads executed.
  2. When an instance is a Secondary and it eventually logs a "slow" (as per slowms threshold) mirrored read, add a field in the logs to clearly show the origin of this call like "mirrored: true"

These changes should be very helpful when troubleshooting, and the suggested changes should be added to both v4.4 and v5.0.


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