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

[Friedrich Weber <f.weber@proxmox.com>]

When using Windows VMs with KRBD, it can be necessary to pass the rxbounce
option to `rbd map` to avoid "bad crc/signature" warnings in the journal,
retransmits and degraded performance. A first attempt to allow this [0] added
an rxbounce storage option, but Thomas pointed out that this might nudge users
towards creating two storages pointing to the same Ceph pool (one for Linux VMs
that do not need rxbounce, one for Windows VMs that need rxbounce), which we
should discourage.

The cleanest solution from the user POV would for us to pass rxbounce only when
mapping disks of VMs with Windows OS type, but this is not straightforward to
implement, as pve-storage currently does not have knowledge about the "owner"
of the volumes it manages. Fabian and Thomas suggested an approach to pass
well-defined "hints" from e.g. qemu-server to pve-storage [0], one of which
could be the guest's OS type, and based on which the RBD plugin could decide
whether to pass rxbounce or not. We'd need to take care to avoid too much
coupling between qemu-server and pve-storage, though.

Here I took a first attempt at implementing such "hints" after an off-list
discussion with Thomas. This RFC is intended as a base for further discussion
whether we want hints in general, and if yes, whether we want them in this
form. It should definitely not be merged like this and I tested it only
superficially.

For now, PVE::Storage::activate_volumes and PVE::Storage::map_volumes accept
"hints" an optional hash reference with a JSON schema (which they validate).
These hints are then passed to Plugin::{activate,map}_volume. PVE::Storage also
provides a subroutine to check whether the storage stack supports a particular
hint, callers (such as qemu-server) can use that to determine whether they can
pass a certain hint. Thomas suggested (something like) that so we don't
necessarily always have to bump qemu-server when we introduce a new hint in
pve-storage.

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).

- But sometimes, it might be fine to not pass hints to activate_volumes because
  we immediately deactivate, e.g. in QemuServer::restore_vma_archive, but only
  because the only current hint 'guest-ostype' is not relevant in such cases -
  might not be the case for future hints?

- 'guest-ostype' might be the only hint we need for now and can be taken from
  the guest config, but can we be sure that future hints are also going to be
  derived from the guest config? Is it OK to have generate_storage_hints only
  take the guest config as well?

- minor, but still: with this approach, rxbounce would also be passed for the
  EFI disk and TPM state of Windows VMs. This is likely not needed, but probably
  OK for a first version?

- should we already pass hints when activating volumes in pve-container too,
  even though they are empty?

- is it even OK to pass the hints only to {activate,map}_volumes? What if an
  external storage plugin needs the hint also when deactivating a volume? Should
  we already add a $hints parameter to other plugin methods as well, to avoid
  having to do that (and another APIVER bump) later?

- With hints being an argument to activate_volumes which takes a list of volumes,
  the hint applies to all volumes in the list. Is this OK API-wise? For the
  guest-ostype hint, this only makes sense if all passed volumes belong to the
  same guest and not several different ones (because they may have different OS
  types), which I think is the case for our call sites, but doesn't have to be
  the case?

- bumping the APIVER and docs are of course also needed, but left out of this
  RFC for now.

All of the above can likely be sorted out, but especially modifying all
direct+indirect call sites of {activate,map}_volumes will need some work and
care to avoid subtly breaking things. Before getting into that, I wanted to ask
for opinions about the general approach -- also because on the one hand I want
to avoid "overfitting" the guest hints to the one usecase we currently have,
but also avoid overengineering it to fit hypothetical usecases we don't even
know about yet.

What do you think?

[0] https://lore.proxmox.com/pve-devel/20241025111304.99680-2-f.weber@proxmox.com/T/
[1] https://lore.proxmox.com/pve-devel/0fdd8f93-8ef6-4c83-92ab-8e86d5386343@proxmox.com/

pve-storage:

Friedrich Weber (2):
  plugin: map/activate volume: allow callers to pass hints
  plugin: rbd: pass rxbounce when mapping Windows VM disks

 src/PVE/Storage.pm           | 34 ++++++++++++++++++++++++++++++----
 src/PVE/Storage/Plugin.pm    | 17 +++++++++++++++--
 src/PVE/Storage/RBDPlugin.pm | 13 +++++++++----
 3 files changed, 54 insertions(+), 10 deletions(-)


qemu-server:

Friedrich Weber (1):
  fix #5997: qemu: pass guest-ostype hint when activating volumes

 src/PVE/API2/Qemu.pm             | 18 ++++++++++---
 src/PVE/QemuConfig.pm            |  1 +
 src/PVE/QemuServer.pm            | 44 ++++++++++++++++++++++++--------
 src/PVE/QemuServer/Cloudinit.pm  |  3 ++-
 src/PVE/QemuServer/ImportDisk.pm |  4 ++-
 src/PVE/QemuServer/OVMF.pm       |  4 ++-
 src/PVE/QemuServer/QemuImage.pm  |  5 ++--
 src/PVE/QemuServer/RunState.pm   |  3 ++-
 src/PVE/VZDump/QemuServer.pm     |  6 +++--
 9 files changed, 66 insertions(+), 22 deletions(-)


Summary over all repositories:
  12 files changed, 120 insertions(+), 32 deletions(-)

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