[dm-devel] [LSF/MM TOPIC] block, dm: restack queue_limits

[NeilBrown]

On Mon, Jan 29 2018, Mike Snitzer wrote:

> We currently don't restack the queue_limits if the lowest, or
> intermediate, layer of an IO stack changes.
>
> This is particularly unfortunate in the case of FLUSH/FUA which may
> change if/when a HW controller's BBU fails; whereby requiring the device
> advertise that it has a volatile write cache (WCE=1).
>
> But in the context of DM, really it'd be best if the entire stack of
> devices had their limits restacked if any underlying layer's limits
> change.
>
> In the past, Martin and I discussed that we should "just do it" but
> never did.  Not sure we need a lengthy discussion but figured I'd put it
> out there.

So much "yes"!!
Just a notifier chain would probably do.

I would like the notification to support changing the size of the device
too.
I see this as being two-stage.
1/ I'm going to change the device size to X - are you all OK with that?
2/ Device size is now X.

That allows md and dm to check that filesystems aren't going to get mad
when devices are made smaller, and can adjust (if they want to) when
devices get bigger.

Thanks,
NeilBrown

>
> Maybe I'll find time, between now and April, to try implementing it.
>
> Thanks,
> Mike
>
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