What exactly is happening when creating synthetic full backup?

[FireExit]

This backup job is creating synthetic full backup (25% done) [fast clone] - 2 hours 53 minutes so far.

I thought fast clone (Windows/ReFS) would be just making a metadata file with links to the existing blocks on disk, without copying any blocks of data around, so it would be busy with CPU and disk activity.

CPU use is <20% and that's mostly my remote control session, Veeam processes are barely showing up.
Memory use is <20%.
Disk activity is almost nothing, 75Kb/s from Veeam SQL Server templog.ldf.
Disk response time is 0ms, queue length is 0.01, nothing is busy with disk IO.
Network activity is 50k/sec out to my RMM remote control, and almost nothing to VMware.
There's no other backup jobs, and the server doesn't do anything else, just a physical host with SQL, Veeam B&R and a few disks.

It is progressing, it's moved from 25% to 29% as I'm watching so it hasn't crashed.

... what is the Synth Full process actively doing, and what might speed it up? (I haven't made a ticket - it's a weekend, not broken and feels like a waste of support time, I'm mostly curious about the internals).

[david.domask]

Hi FireExit,

> I thought fast clone (Windows/ReFS) would be just making a metadata file with links to the existing blocks on disk, without copying any blocks of data around, so it would be busy with CPU and disk activity.

More or less correct, but it's not making just a metadata file, the file itself will have references to datablocks from the file it's sharing blocks with -- this is "part" of the file itself, and the file system will recognize that the file is composed of its own blocks + data blocks that are being shared.

Microsoft's animation on how Fast Clone works may help with understanding the process. As for the speed you posted, how big of a backup are we discussing here? 2+ hours is a bit long for a Fast Clone'd synthetic full but depending on quite a few factors it can take longer for some operations, but 2+ hours is a bit long.

Start with a Support Case and let Support review the environment. Be sure to include logs for Support to reivew (use the 1st radio option, select one of the affected jobs for Support to review).

Please share the case number here once created. Support has some tools for establishing a base line test for the Fast Clone performance outside of Veeam and can check for other elements that may be a factor.

[FireExit]

Thank you for the documentation link; the synthetic full is 3.3TB and it ended up taking 30 hours 26 minutes to finish.

I have opened support case #07444150