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I recently right-sized the data and log files for an RDS database, leaving just a few GB of unallocated space. Shortly thereafter, something allocated, then deallocated, 1 GB of the margin of error. What might have caused this?

All of the databases - including tempdb - are either tiny (MB) or at exactly the size they've been set; they add up to 197 GB, leaving 3 GB free on a 200 GB instance. And yet:

Free space dipped from 3 to 1.5 GB, then returned

Clearly, if this happens again in a slightly greater magnitude, there will be a problem. For now, I can simply shrink one of the database files to give a larger margin, but I'd like to understand what actually happened.

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AWS support came back with a comprehensive answer. In this case, it was transaction log backups, being written to the SQL disk before offloading to S3. There are a few more things which can consume space, mostly temporarily. As their tech outlined:

As a best Practice, I would recommend to keep at around 5% free space for similar processes (Log Backups, Error Log Files, Trace Files, etc), however this may vary based on high transactional volume or bulk data upload process , which may need high disk storage temporarily.

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