No Impact on a VM when Using the fstrim Command to Save Space

Impact of `fstrim` Command on VM

Safety and Normal Operation

The fstrim command is generally safe to run on a VM and is considered a standard maintenance operation. It’s designed to be non-destructive and only discards blocks that are already marked as unused by the filesystem.

Potential Impacts During Execution

Performance Considerations

Temporary I/O increase: fstrim generates disk I/O activity while scanning and discarding unused blocks
Brief performance degradation: The VM may experience slower disk response times during the operation
CPU usage: Minimal CPU impact, primarily I/O bound operation
Duration: Depends on disk size and amount of unused space (typically minutes for a 93GB disk)

What Actually Happens

fstrim sends TRIM/DISCARD commands for unused blocks to the underlying storage
In your case with QCOW2, it allows the host to reclaim unused space from the image file
Only affects genuinely unused blocks - no data loss occurs
The filesystem remains fully accessible during the operation

Best Practices for Running `fstrim`

Run during low-activity periods if possible (though not strictly necessary)
Start with a specific mount point: sudo fstrim -v / before using -a for all mount points
Monitor the output - it will show how much space was trimmed
The VM remains fully operational - no downtime required

Expected Outcome for Your Situation

Should help reduce the rootdisk.qcow2 file size on the host
May free up some of that 92.6 GB usage if there are unused blocks within the guest
No negative impact on VM data or applications
Worst case: No space is reclaimed (if all blocks are genuinely in use)

The command is reversible in the sense that if space is needed again, the QCOW2 file will simply grow back as required.