Updates disappear from upd list

[Matan Tennenhaus]

Hi, a question about updates lists management.

I am observing the content of the update lists during reconciliations, where updates disappear from the update list without eviction, even though they were selected in a previous reconciliation.

My scenario: Without evictions, I insert some key, and after do updates & reads.

In the first reconciliation, all updates belong to to a corresponding WT_INSERT struct. Afterwards no eviction occurred so updates append to those lists, and the next reconciliation previously selected update is missing from the update list it belonged to.

1) How can this happen? I assumed that if an update in inserted into the update list it can only be removed when modify structure is freed (page is evicted).

2) My problem is that for updates selected in reconciliation i added a metadata which i dynamically allocated, so if an update disappears(=freed) without me freeing the added metadata, I get memory leak. So for a solution to my problem, if I knew where update can be freed like the scenario above, I would add code to handle my metadata.

2.1)Maybe if i get reference to the update somehow it can also prevent this scenrio?

3) Maybe its a bug? I am using tag mongodb-7.0.2 (WT 11.2.0).

Much appreciate.

Matan.

 

[Chenhao Qu]

Hi Matan,

There is another place in the code apart from reconciliation that we can remove updates from the update lists. Check out __wt_update_obsolete_check. It removes all the obsolete updates that are globally visible.

Cheers,

Chenhao

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[Matan Tennenhaus]

Thanks!

ב-יום רביעי, 18 בספטמבר 2024 בשעה 02:04:53 UTC+3, Chenhao Qu כתב/ה: