[pve-devel] [RFC qemu-server/storage 0/3] fix #5779: introduce guest hints to pass rxbounce flag to KRBD

[Friedrich Weber]

On 24/10/2025 14:27, Friedrich Weber wrote:
> [...]
> Obstacles I faced so far:
> 
> - The biggest obstacle is that we need to update all callers of
>   activate_volumes to pass guest hints where possible. This means that optimally
>   all callers should be able to generate the guest hints. Right now, there is
>   only the 'guest-ostype' hint which is taken from the VM config. Currently, this
>   is not always available to the caller of activate_volumes, sometimes
>   some extra work/refactoring would be needed to get it (e.g. see
>   PVE::QemuServer::QemuImage::convert or PVE::QemuServer::clone_disk),
>   so this needs quite some code changes, which I have not done in all cases in
>   this RFC.
> 
> - There are also some indirect callers of activate_volumes, e.g. via
>   PVE::Storage::abs_filesystem_path or PVE::Storage::storage_migrate -- these
>   would also need to be extended to accept hints (not done in this RFC)
> 
> - Initially, to avoid having to modify all (direct+indirect) callers of
>   activate_volumes, I thought I could pass the hints only at the few "relevant"
>   call sites (i.e., when starting a VM), but then noticed that volumes may be
>   activated by an action unrelated to a VM start (e.g. a clone), then stay
>   active, and not be re-activated by a VM start. So if e.g. we do not pass the
>   hints on clone, the KRBD volume would be mapped without rxbounce, stay active,
>   and when starting the VM, a user could run into the original problem again.
>   So we can't get away with only passing hints to the few relevant call sites,
>   and actually need to pass them everywhere (where possible).
> 

Thomas and I discussed this point off-list:

- to clarify: if a Windows guest volume was mapped with KRBD without
rxbounce (e.g. by a clone where the activate_volumes caller doesn't pass
$hints) and doesn't get unmapped, and then a VM start activates the
volumes again (passing $hints this time so we'd like to pass rxbounce),
RBDPlugin::map_volume will early-exit because the volume is already mapped:

sub map_volume {
    my ($class, $storeid, $scfg, $volname, $snapname) = @_;

    my ($vtype, $img_name, $vmid) = $class->parse_volname($volname);

    my $name = $img_name;
    $name .= '@' . $snapname if $snapname;

    my $kerneldev = get_rbd_dev_path($scfg, $storeid, $name);

    return $kerneldev if -b $kerneldev; # already mapped

    [...]
}

... which is the VM will just use the guest volume without rxbounce and
the user can run into the issue.

- we discussed whether, to avoid this, we could apply the rxbounce
option "on the fly" to an already-mapped volume. I looked a bit [1] and
didn't see any way to apply rxbounce to an already-mapped volume.
Calling `rbd map` again apparently just maps the volume a second time
which doesn't sound like a good idea, and an `rbd unmap` followed by an
`rbd map` (with rxbounce) is likely not safe either?

[1] https://docs.ceph.com/en/reef/man/8/rbd/