(The Curious Case of… used to be part of our bi-weekly newsletter but we decided to make it a regular blog post instead so it can sometimes be more frequent. It covers something interesting one of us encountered when working with a client, doing some testing, or were asked in a random question from the community.)
Last week I was emailed a question about weird messages in someone’s error log on an Azure Managed Instance, similar to these:
[DbId:12]. Removing xdes id: 0000:8112c9d0 from aborted xdes map. [DbId:12]. Removing xdes id: 0000:8112b45e from aborted xdes map. [DbId:12]. Removing xdes id: 0000:8112b403 from aborted xdes map.
First off, an XDES is a data structure (and C++ class) in the Storage Engine that represents a transaction; it stands for ‘transaction descriptor’. The XDES ID is the same one that you see in the Transaction ID column in the output from the fn_dblog function for examining transaction log contents (see here for examples).
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The messages are benign and are to do with the internal workings of the Accelerated Database Recovery feature, that allows instantaneous rollback of transactions through a clever versioning mechanism.
The aborted transaction map (ATM) contains a list of transactions that have rolled back (internally called ‘aborted’), but their effects have not yet been ‘removed’ from the database. The ATM is used:
- when a query reads a row, to see whether the current version of the row is from a transaction that subsequently rolled back, and so a previous version of the row should be used
- when a query is updating a row, to see whether the current version of the row is from a transaction that subsequently rolled back, so the updated row should overwrite the current version of the row
When the effects of a rolled-back transaction no longer need to be considered, the transaction ID is removed from the ATM.
You can read a great post that explains the internals of Accelerated Database Recovery here.
One thought on “The Curious Case of… the aborted xdes map”
Thanks for sharing your findings.