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Status Not under consideration
Workspace Db2
Components Monitoring
Created by Guest
Created on Nov 10, 2021

An analog of LOG_HADR_WAIT_TIME monitor element is needed at the agent level

Sometimes it's very useful to understand HADR influence on the transaction performance.

We do have a couple of related elements - LOG_WRITE_TIME and LOG_HADR_WAIT_TIME at the whole database level, but their values can't help us to understand, if HADR communications significantly slow down particular statements, transactions, workloads, etc.

The only more or less meaningful attempt to calculate HADR influence on user transactions is like as below for whole database:


SELECT

100 * D.LOG_DISK_WAIT_TIME * T.LOG_HADR_WAIT_TIME / (T.LOG_WRITE_TIME + T.LOG_HADR_WAIT_TIME)

/ D.TOTAL_RQST_TIME AS HADR_OVERHEAD_PST

FROM TABLE (MON_GET_DATABASE (-2)) D

JOIN TABLE (MON_GET_TRANSACTION_LOG (-2)) T ON T.MEMBER = D.MEMBER;


But it looks like an "average hospital temerature" - TOTAL_RQST_TIME is a total time for all requests including all non data updating requests, where HADR influence is 0.

Whereas such an influence on data modification statements may be very significant, but hidden because of lots of non data updating requests.


The ideal solution whould be having some new "LOG_HADR_WAIT_TIME_agent" monitor element under existing LOG_DISK_WAIT_TIME showing the HADR logging overhead only at the agent level.

Needed By Yesterday (Let's go already!)
  • Admin
    Michael Roecken
    Nov 11, 2021

    Thank you for your idea.

    Summary: there is already reporting of HADR overhead at the database level via log_hadr_wait_time (in comparison to log_write_time). This can be roughly translated to agent level, via the following example:

    • if log_write_time is 10 and log_hadr_wait_time is 10, the system would perform exactly as if you are using disk that is half of the speed, ie. as a system not using HADR with log_write_time of 20 .. if log_write_time is 10 and log_hadr_wait_time is 1, then HADR is just adding 10% to the log write cost and the system is identical to a non-HADR system with log_write_time of 11

    But it is not going to be a precise break down, but adding such is not something the business wants to invest in at this time.