[pve-devel] [PATCH storage 1/2] fix #5779: rbd: allow to pass custom krbd map options

[Friedrich Weber]

On 30/10/2024 17:49, Friedrich Weber wrote:
> [...]
> 
> Yeah, I see the point.
> 
> Of course, another alternative is enabling `rxbounce` unconditionally,
> as initially requested in [1]. I'm a hesitant to do that because from
> reading its description I'd expect it could have a performance impact --
> it's probably small, if any, but this should probably be checked before
> changing the default.
> 

I took another look at this: When rxbounce was first introduced, there
was a discussion whether krbd could enabled automatically switch to
"rxbounce mode" if needed [0]. I asked upstream whether this seems
realistic [1], and they responded it's very unlikely to happen.

So the cleanest solution from a user point of view might be if PVE
automatically passes rxbounce only when mapping disks of Windows VMs.
But as Fabian points out [2], this would require some way to pass this
information to the storage layer.

Of course always passing rxbounce is still an option. Upstream confirmed
there is a theoretical performance impact, but it might be negligible in
practice [0]. So if benchmarks show the impact is indeed negligible, we
could go for that route.

But even with benchmarks done carefully, there is a chance that they
fail to catch a performance impact on some types of workloads. So in
order to not disturb setups that currently work fine without rxbounce, I
have a slight preference for only passing rxbounce when needed, even if
that requires some architectural changes.

[0]
https://lore.kernel.org/all/894a36483c241e0cc5154e09e8dd078f57a606d5.camel@kernel.org/
[1]
https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/message/ZSXCXPTLMQPV27Y7I375OBR7CN56LDGH/
[2]
https://lore.proxmox.com/pve-devel/1234079298.5156.1730294987348@webmail.proxmox.com/