Sync not called during OS Flush or O_DIRECT?

[benduncan]

@pojntfx and team, great work on Go-NBD!

I'm playing around with an implementation, however notice a bug that I do not receive Sync() events from my backend after an OS disk flush/sync.

func (b *VblockBackend) Sync() error {
slog.Info("Sync event fired")
...
}

If using

dd if=/dev/nbd3 of=/dev/null bs=32K iflag=direct

Or copying/editing files on the mounted /dev/nbd* block device and running sync, this is not triggered in the go-nbd server by calling my plugins Sync() function.

Suggestions?

I'm using Ubuntu 25.04 and go version go1.24.3 linux/arm64 and mounting with nbd-client (nbd 3.26.1)