Unless the VM has a mechanism to inform the host of sync writes, all writes should be treated as sync writes by default. 24.10 VMs seemed to treat all writes as sync writes, but 25.04-RC1 does not. I observed this by monitoring activity on my LOG device, but I'm sure there are better ways to check.

My steps to reproduce:
Boot a linux and execute the following 2 commands to write a test 1GB of random data.
dd if=/dev/urandom of=/tmp/disk bs=1M count=1024
dd if=/dev/urandom of=/tmp/disk oflag=sync bs=1M count=1024

Desired result:
Best case, the second command should produce sync writes on the host OS indicating that the VM has informed the host of sync writes only when sync IO is necessary.

Typical case, both commands produce sync writes, indicating the host doesn't know what is and isn't a sync write, thus it's treating everything as a sync write to guarantee data integrity within the VM.

Current result:
Neither command appears to produce sync writes, leaving VM data integrity at risk.

Workaround:
Manually setting a VMs zvol to sync always will mitigate this bug.

Session ID: 3c06b8f3-50a9-1887-5072-792ff163408e