Delta Table Maintenance for Mirrored DB Tables

Hi JuliaMeneley ,

Short answer:
I would not treat mirrored tables the same way as regular Lakehouse Delta tables for manual maintenance.

For regular Lakehouse Delta tables, Fabric supports maintenance operations such as OPTIMIZE, V-Order and VACUUM.

However, for mirrored tables, I would treat the mirrored output as Fabric-managed unless Microsoft confirms otherwise.

Microsoft documentation for Lakehouse table maintenance:
https://learn.microsoft.com/en-us/fabric/data-engineering/lakehouse-table-maintenance

Why this likely happens:
Mirroring is a managed replication feature. Microsoft describes Mirroring as continuously replicating data into OneLake and automatically keeping the data in sync. That makes the mirrored table different from a normal Delta table that your Lakehouse owns and writes to directly.

Microsoft documentation on Mirroring:
https://learn.microsoft.com/en-us/fabric/mirroring/overview

Also, if you are accessing the mirrored table through a Lakehouse shortcut, the shortcut is just a reference to data without copying it. So the table appears in the Lakehouse, but the underlying files are not physically owned by that Lakehouse.

Microsoft documentation on Lakehouse shortcuts:
https://learn.microsoft.com/en-us/fabric/data-engineering/lakehouse-shortcuts

Maintenance commands such as OPTIMIZE and VACUUM are not just read operations. OPTIMIZE compacts/reorganizes Delta files, and VACUUM removes obsolete files that are no longer referenced by the Delta log. So they require write/delete access to the underlying Delta storage.

Your result also seems consistent with this:

numFilesAdded: 0
numFilesRemoved: 0
totalFilesSkipped: 1

That suggests Spark did not compact or rewrite the mirrored table files.

What I would do:
If you need full control over table maintenance, I would create a downstream curated Delta table:

Mirrored table
→ read from it
→ write to your own Delta table in the Lakehouse
→ run OPTIMIZE / VORDER / VACUUM on that curated table

Practical rule:

• Regular Lakehouse Delta tables: maintain them yourself.
• Mirrored tables / shortcuts: treat them as Fabric-managed or source-owned.
• If you need custom optimization: create a curated Delta copy and maintain that.

Best regards,
Solutions Architect · Microsoft Fabric Specialist · Parchitect

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