Re: Applying Throttle Block Filter via QMP Command

[Alberto Garcia]

On Wed, Jan 08, 2025 at 06:45:54PM +0900, Henry lol wrote:
> I want to apply a throttle block filter using the QMP command, but
> it doesn't seem to work as the I/O performance remains the same.
> 
> Are there any additional steps I need to follow?
> I predefined the throttle-group object and block device in the QEMU
> parameters and then used the blockdev-add QMP command to apply the
> filter, as described in the link
> - https://github.com/qemu/qemu/blob/master/docs/throttle.txt#L315-L322
> 
> Additionally, I’ve confirmed that the filter works well when defined
> in the QEMU -drive parameter instead of using the QMP command.

Can you summarize the commands that you are using?

Simply adding the filter with 'blockdev-add' is not enough, that
creates the backend (the "host" part, i.e. how the block device is
actually emulated) but you also need a frontend (the device that the
guest VM can see, i.e. a SCSI hard drive, an SD card, etc.).

The -drive parameter creates both things (frontend and backend).

See here: 
https://www.linux-kvm.org/images/3/34/Kvm-forum-2013-block-dev-configuration.pdf

Berto