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Status Not under consideration
Workspace Db2 for z/OS
Created by Guest
Created on Sep 27, 2023

RELATED IDEAS

Improved LATCH when index splitting

[Current Product & Version]
-Db2 12 for z/OS
-IBM Tivoli OMEGAMON XE for DB2Performance Expert on z/OS v5.5
-z/OS v2.3
-WebSphere Application Server v9 for z/OS

[Environment]
Data sharing in three-way sysplex environment

[Problem]
Our customer is using WAS for z/OS and DB2 for z/OS on their mission critical system.
Several times our customers experienced slow transaction responsiveness and timeouts, severely impacting their business.
I opened CASE(TS013070610/TS013085686）to resolve this problem, but these CASEs does not identify the cause of the problem and does not currently resolve the problem.

This CASE explained that some read thread held S page latch waiting, it could cause the index split thread to wait on the page latch, so many transactions became wait or time out.
Unfortunately, it was not possible to identify the read thread holding S-LATCH for a long time from materials such as Db2 logs.

As a side note, obtaining SLIP dumps using SLIP traps is not acceptable due to concerns about reduced transaction response.
We believe that product improvements are necessary to identify the cause of this problem.

[Workaround(Bypass)]
none

[Proposal/Idea]
If a page in an index split is a higher level page, LATCH will be held for all related lower level pages.
In that case, there is a concern that the number of transactions that will be affected will increase, leading to an increase in the number of transactions that will result in poor response.

When an index split occurs, instead of holding LATCH until the latch locks on all pages are released, reduce the range of pages that hold LATCH sequentially and improve it so that fewer transactions are affected by waiting for LATCH.

[Benefits/Value]
Our customers can identify the cause of this problem, consider a solution, and expect normal and stable business operations.

Needed By Quarter
  • Admin
    Janet Figone
    Reply
    |     Apr 8, 2024

    Dear Noma-san,

    Thank you for submitting this enhancement request. Our Db2 for z/OS team evaluated this item carefully and decided not to pursue further. We understand the possible impact from index page split, however, we have been already holding the index page tree latch at the limited area that will be affected by the split. Limiting the latch area further will cause the risk of the data integrity issues. Similarly, we have also reviewed the possibility to reduce the duration of holding the latch, and concluded we have already limited the duration and optimized the process.

    In the described scenario, if the thread is waiting for a page latch or a p-lock, the waiter’s accounting traces will have more detail information for wait time starting in Db2 V13 and should be quicker than going through Db2 log records. We have also improved the index split diagnosis in Db2 V13 to publish the exception record (IFCID 369) when the split takes longer than 1 second.

    We hope additional diagnostic information will be able to help you to speed up the problem analysis on Db2 wait time related with a page latch and index page split.


    We appreciate your input to the Db2 for z/OS development team. And we hope that you will continue to submit enhancement suggestions for improvements as customer feedback is a key component to shaping the future direction of Db2 for z/OS.

    Sincerely,

    The Db2 for z/OS team

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