Why is log reserve negative in the SQL Server compensation log when using fn_dblog command
The function is technically undocumented, so I can't find any official word on this.  However, my understanding is that those &quot;compensation&quot; rows are related to a rollback.  A rollback is essentially erasing some previous log records, so it frees up some space in the transaction log.  The negative values are saying &quot;give this much free space back to the log file.&quot;
You can observe this by performing a demo like this:
USE [master]
GO
DROP DATABASE IF EXISTS LogUsageTest;
GO
CREATE DATABASE LogUsageTest;
GO
ALTER DATABASE [LogUsageTest] 
SET RECOVERY SIMPLE 
WITH NO_WAIT;
GO
USE LogUsageTest;


CREATE TABLE dbo.A
(
    Id int IDENTITY(1,1) NOT NULL,
    Something int NOT NULL

    CONSTRAINT PK_A PRIMARY KEY (Id)
);
GO
CHECKPOINT;

That sets up a database in the SIMPLE recovery model with one table in it, then clears the log (with a checkpoint).  Here's the baseline transaction log usage:

Now I'll run an insert inside a transaction:
BEGIN TRANSACTION;
INSERT INTO dbo.A
    (Something)
SELECT
    v1.number
FROM master.dbo.spt_values v1;

We've gone from 5.5% usage to 13%:

And now we'll roll that insert back:
ROLLBACK;


And we've gone from 13% down to 8.9%.
You might notice that the compensations don't free up all the log space used by the insert.  In my test, the inserts reserve 178 bytes, while the rollbacks free up -74 bytes.
I think this is because the log reservations are extra big in case of page splits, which don't get rolled back.  But that's just an educated guess.